Is a non-compete enforceable if your previous employer did not pay you?

Noncompetes are generally legal, but they have to be "reasonable"--not too long, not wide a geographic area. What is reasonable depends on the context and the industry; if you can sell insurance statewide, for example, it might be found to be reasonable to keep you from competing at least for a period of time (6 months?). Generally speaking, if you believe the noncompete is too broad or long, you can either go to court to get a judge to rule on that (which means your paying for a lawsuit) or you can go to work somewhere and if your new job doesn't fire you when they hear there's a noncompete, the next step is for the former employer to try to sue you to enforce the clause.

Given that there was a noncompete, you probably can't sue your former boss for calling the new company--even if he was wrong and the noncompete would ultimately be found unenforceable, he was not unreasonable in thinking he could call to enforce it.

Since your former boss did not pay you and you presumably only signed the noncompete in order to get the job (and get paid), you might have grounds to argue that the agreement is void--that his failure to pay and breach of the agreement renders it unenforceable.

You may sue for wages owed.

This could be very complex and fact specific--it depends on the exact language of the agreement and again, and issues of what is reasonable in your industry. You should take a copy of the agreement and seek a consultation with an experienced local employment law attorney.