This has happened to me twice in the last two days. The first time I had to uninstall OneDrive and download and install it again. There were many sync errors to correct. Today, I discovered a fix which was much easier and no sync problems. See steps below.
How can OneDrive just disappear with no warning that it is not protecting my files? Why is this happening?
Steps I took today:
...I browsed around a bit more and found OneDriveSetup.exe in the %LocalAppData%\Microsoft\OneDrive\Update folder. I ran this file and I got the dialog box indicating that OneDrive was being prepared for first use. When it completed, it kept the settings for both my personal and OneDrive for Business. The only hiccup was that the file I was working on over the last two days was listed as a conflict that I needed to resolve.
As much as this annoyed me, I think this shows how much better the OneDrive client is than it used to be....