Unfortunately, that's not enough info for me to know what happened.
What did you do immediately before the error? A screen shot may help us determine what Toad is doing.
Is this the first time the error appeared? Does Toad work with other databases?
See also my comments here about getting a full error log.
If Toad has been working for a while but is now suddenly giving this error, you may need to reset your settings. There are steps to do that here.
-John