Hi, I stupidly changed my admin account (not the built-in one) into a
standard user account. I have previously enabled the built-in
administrator account so I thought that I will be able to login as
administrator using that built-in account. But the welcome screen now
does not show the built-in administrator account and I have no access to
any admin account.

I have tried using safe mode but no admin account showed up in the
welcome screen.  I cannot even run cmd in safemode as an administrator.
A dialogue box will appear saying that "User Account Control - to
continue, type admin password...". However, there is no prompt for me to
type the admin password and the ok button has also been disabled.

Please help!