This is the same thing happened to me. BD stopped working after i did a Program Update.
It seems BD had deleted a Registry Entry. Heres how i fixed it,
Open Registry Editor (Start > Run > Regedit.exe)
Open:
CODE
HKEY_LOCAL_MACHINE\SOFTWARE\BitDefender\BitDefender Threat Scanner
If u have the same problem like mine then there will be no "EnginesFolder" entry but there will be a "EnginesFolder2" entry.
So, rename "EnginesFolder2" to "EnginesFolder" and restart the PC for the changes to take effect.
I hope this solves ur problem.