Sorry about all the forum problems recently.
Looks like it may now be fixed.
I found a repair php file on the SMF Forum that seems to have worked (and detected one possible path error)
It has certainly restored my Admin access which is a big relief.
I changed the cookie name just in case, so you'll have to delete your cookies in ordrer for it to work again.
Still no idea why it went bust, I didn't touch anything, but apparently it's a fairly common problem with SMF forums.
Any further issues let me know.
Thanks
Dave.