Didn't bother changing themes as suggested in other replies to a similar problem because that fix was directed at those who had recieved the error after upgrading WordPress.
Deleting the DB cache plugin did nothing. So I deleted the other cache plugin (W3 total cache) I had installed. That generated another error msg. Can't quite remember what that error said but soemthing to do with advanced-cache.php.
Next I deleted files in the tmp folder and that did nothing.
I then deleted the db.php, advanced-cache.php and object-cache.php and I was able to get back into my site and admin area.
I reinstalled the W3 total cache but not the DB cache plugin. This plugin has not been updated to work on WordPress version 3. x. The 'reloaded' and 'reloaded fix' versions of this plug-in are also dubious in my eyes. Check the comments before installation. I never did.
I reckon deleting the 3 files would have solved the problem in the first place, for me at least.