I changed my account name through Regedit and now I can’t sign into my account.

Hi Wasupol. I'm Greg, 10 years awarded Windows MVP, specializing in Installation, Performance, Troubleshooting and Activation, here to help you.

Yeah, that's the User Profile folder name, which is different from the User Account name changed as shown here:
http://www.thewindowsclub.com/change-account-us...

The User Profile Folder has never been able to be changed without causing Windows to stop booting, until MVP Shawn Brink published a method here that is safe:
https://www.tenforums.com/tutorials/89060-chang...

To undo the regedit and get Windows to start again you need to enter the Repair Mode or boot WIndows media to do a System Restore:
https://www.tenforums.com/tutorials/2294-advanc...
https://www.microsoft.com/en-US/software-downlo...

If System Restore doesnt' work then the only other repair is to access the Command Prompt from Repair Mode or Bootable Media to mount the registry to edit it outside of Windows, then reload it back into Windows, as shown here:
https://www.winhelponline.com/blog/edit-registr...

Change the registry back to it's default before you changed it, reload registry, restart

Based on the results you post back I may have other suggestions if necessary.

Back in the morning, Central US time.

______________________________________________

Standard Disclaimer: There are links to non-Microsoft websites. The pages appear to be providing accurate, safe information. Watch out for ads on the sites that may advertise products frequently classified as a PUP (Potentially Unwanted Products). Thoroughly research any product advertised on the sites before you decide to download and install it.

Thanks for the instruction Greg. I couldn’t complete the process to undo the regedit because I have forgotten my password. I’m stuck at the step shown in picture below.

So I clicked on “Forgot your password or don’t see your account” button as shown below

Then, it brings me to the following page shown in picture below.

Then I clicked the “Restart” button, and the Windows bring me back to the same temporary account which pops up the same “We can’t sign in to your account” window again.

I don’t know how to retrieve my forgotten password for the admin account (which I’m not sure I ever owned one since my Window 10 (genuine) is installed for me right from the Thinkpad’s/my hardware’s manufacturer).

Please give further instruction to solve this.

They do not make it easy when you forget your password, or else anyone could say they're you and get into your account. You should first try changing the password.

All possible options for password recovery are in Microsoft's official Support policy here:
https://docs.microsoft.com/en-us/windows/securi...

This goes a lot easier if you set up secondary email and phone number to get a code to change the password or recover your account here: https://support.microsoft.com/en-us/help/12428/...

For a Local Account set up test questions on the sign-in screen as shown here:
https://www.windowscentral.com/how-add-security...

If the password cannot be made to work I will show you how to replace or repair the account so you can still get into Windows, or if necessary reset or reinstall Windows.

This requires accessing the Command Prompt using bootable media which is the only method that doesn't require a password. If you don't want to do that then you can try now to Reset Windows 10 from the Sign in Screen as shown in this video:

https://www.youtube.com/watch?v=Zonjyd2L_GQ

To replace or repair the old account go to another PC to create bootable media following the steps toward the bottom of the download page here in the section "Using the tool to create installation media:"
https://www.microsoft.com/en-US/software-downlo... Uncheck the box for Recommended Settings to choose the exact version and bit rate for the target PC only.

Insert media, boot it by powering up PC while pressing the BIOS Boot Menu Key: https://www.sysnative.com/forums/hardware-tutor...

If the media won't boot you may need to enter BIOS/UEFI Setup (pressing key given in chart in link above) to turn off Fast Boot or Fast Startup first.

Choose the boot device as a UEFI device if offered, on second screen choose Repair Your Computer, then Advanced Troubleshoot Options to open Command Prompt to create a new WIndows 10 Local Administrator account as shown here:
https://www.isumsoft.com/windows-10/3-ways-to-c...

Sign into the new account, test it works and resolves the problem(s), if so you can access or move your files over at C:\Users.

And if all that fails from bootable media choose Install Now, then Custom Install, then at the drive selection screen delete all partitions down to Unallocated Space to get it cleanest, click Next to let it create needed partitions and start install - this makes it foolproof.

Everything needed to get the best possible Clean Install is here: http://answers.microsoft.com/en-us/windows/wiki.... It is a better install than any amount of money can buy and a great learning experience that will make you the master of your PC because you will learn what works best and how to apply it with your own hands.

If you have files that aren't backed up you can use the same bootable media to try to rescue your files using these methods: http://answers.microsoft.com/en-us/windows/wiki...

I hope this helps. Feel free to ask back any questions and keep me posted. If you will wait to choose if I solved your problem, I will keep working with you until it's resolved.

________________________________________________________
Standard Disclaimer: There are links to non-Microsoft websites. The pages appear to be providing accurate, safe information. Watch out for ads on the sites that may advertise products frequently classified as a PUP (Potentially Unwanted Products). Thoroughly research any product advertised on the sites before you decide to download and install it.