I Uninstalled Office365 Now I Can't Login to PC

[BKLLC]

Environment is Dell Optiplex running Windows 10 Pro in a Workgroup. No AD/Domain.

I switched from Office365 to standard desktop Office package.  Office365 app was successfully removed using standard uninstall process (thru Control Panel).

At first, everything seems to worked perfectly without a problem, SMTP/IMAP emails all worked fine on desktop Outlook. However, when the PC came back from sleep mode, users where unable to login again. We reboot the PC, still no user account to login with. One will expect Windows to fall back to default account just as it does when you move a PC from Domain to Workgroup.

How can I regain access into the PC using default Windows built-in user account or create new user?

I have tried booting with Windows DVD media but the C:\Windows\ is not visible.  I can see Dell enabled "Secure Boot" in the BIOS, switching to Legacy mode in BIOS gives message that causes the Internal boot media to become unavailable, therefore no C:\Windows\System32" 

I just want to regain local access to a stand alone Win10 PC.

[#2]

@BKLLC  I Guess what happened here, there was no local account set up with this machine it was set up using that Office 365 emails account that was used as login user/password to this computer, upon deletion & Changing the office 365 that email is no more valid, but you should always get login using the local cached details with same Email account as login and the password as that is the only login and configured users, there are alternative methods and software you could enable local Administrator and try to create a new local standard or Administrator user in that machine.

update here if the case is different.

[#3]

For some reason the old Office365 email and password is not working (not granting access). Either it wasn't cached or it got cleared when the user rebooted the PC before contact me.
I have no problem logging in from another PC to Office365 via web browser using the same ID/pswd, the account is still active on MS side.
I really need to get into this PCs because users committed cardinal sin by saving all their critical work on the Desktop instead of on the provided cloud folder.

I noticed that the drive has Bitlocker enabled as such can't be attached to another PC either.
So please whatever software tool you know that can help grant access to this PCs will be highly appreciated.