Same thing happened to me. I just ended up throwing the check away. Probably depends if it's alot of money and worth pursuing. Do you have prepaid legal? It's not much to be a part of and you get help from an attorney.
If the bank says the check is good and the employer admits to writing the check I would say you have a good case....but ask a lawyer. Might check with state labor board and explain it to them....they might help ?
A check more than 6 months old is no good, so the bank will not honor it. I don't understand why you have a check that's a year and a half old. Assuming you mislaid it, you have a claim for payment that was not made. Because of the small amount, small claims court is your best bet.