The best defence is to make sure they realize that you can't pluck feathers from a frog.
Any weaknesses you have should be offset by your contractual terms (assuming there was a contract).
Since you say you were employed they would have to work very hard to prove malice on your part, especially since they decided to keep you on as long as they did. Any damage they claim you caused should be easy to prove for them, hard to defend against by you if it is to stick, but without more specifics that's unknown.
For the record I have been the plaintiff in a case like this, where an employee decided to blackmail me and turn off a website overnight locking me out of the server. Needless to say that didn't end very well for said employee, not because I'm rich and he was weak but because he was a jerk that did something he never should have done: assume that blackmail is a viable option.
In the end that one was resolved out-of-court by the time the defendants lawyer had a chance to talk some sense into the defendant.
Did you mean employer?
I bet he meant employee.
Without details nobody will be able to give you advice beyond 'get a lawyer' and I fully understand why you don't want to give out any more info here.
The one bit of info I can give you, once you have a lawyer, give him all the information that you can find on this, don't leave anything that you feel is not relevant, a late surprise in a thing like this can be costly.
Best of luck!