From a simple Google search, I've been able to determine that error "x000021" does not in fact exist. Error 0x00000021 does exist, though, but it seems to only apply to Windows 2000 through XP.
If you have another BSOD, copy down the exact error code and search it here.
I have no idea whether or not this issue will re-occur, so the only way to find out is to reboot your machine
If it happens again, boot from last known good config again, and search the exact error code.