In general, with the standard PAR contract, this is considered buyer default. (Standard disclaimer to seek legal advice from an attorney, of course.)
If they checked line 545 on page 10 "seller is limited to sums paid by buyer... as liquidated damages," you can't do much more than keep their hand money. Hopefully it was a significant amount. That's a negotiating point, and one way to mitigate risk if you're worried about the buyer is ask for more hand money, or ask for the hand money to be increased after inspection. Looks like you had no warning here, though.
Hopefully, if you can't persuade them to just extend the closing (in writing, as an amendment to the agreement) and try to work things out, they will sign the REL form (release of hand money) with no trouble. There is also a TER (termination of agreement form), which should be signed by the buyers (the terminating party), to terminate the contract. You (and your agent) would fill out and sign the release form, directing that the escrowed hand money be released entirely to you, the seller, and send that to the buyers (through their agent) for signature.
Once both parties have signed, the buyer's broker turns that in to accounting and you'll get a check in a few days (assuming you're both represented and the hand money was escrowed by the buyer's broker, the typical case in Allegheny County).
As the seller, you're in a bit weaker position if for some reason they refuse to release the hand money. A new law says the deposit reverts to the buyer after a year if there is no agreement *and no pending legal action*. If they refuse for some reason, and you can't work it, out, you'll probably have to go to the magistrate to get it back.
If it's really fallen through, make sure your agent changes the status back to Active right away, so you don't lose any marketing time. Good luck!