I think you're attacking the problem from the wrong angle.
The software is written in PHP? Did you really sabotaged it in such a way as to explicitly require your own intervention? E.g., did it automatically delete the source code that you own, but preserved all the stuff that they own? If it's just a few "eggs" here and there, then they can simply hire someone else to remove those eggs, and be done with it.
If you instead sabotaged their data, or encrypted it in some way, or stored it offsite, then, yes, you're in trouble. Likewise, if they're directly losing lots of money due to your software being suddenly unavailable, which you did on purpose and in bad faith (e.g., without ever informing them of any such condition), then they could likely sue you for damages.
A better approach in the situation would be the legal-offensive side. If they're using your software, without paying you the monthly fee that they've agreed to pay (and for which you have proof of many repeated prior payments, separate from your main employment with them), you should be the one to send them invoices and a cease-and-desist letter, and then if no compliance is reached, sue them, asking the court for a judgement in your favour for the monies owned, and for an injunctive court order for them to stop using your software without providing a payment in return.