Hi,
Roblox has a page about how to recover our account which also includes a couple methods that may help you locate this info.
https://en.help.roblox.com/hc/en-us/articles/360028719931-I-Forgot-My-Username
If you continue to have issues you can contact their support and they may be able to also recover this info for you.
https://www.roblox.com/support
I hope this helps and good luck.