If you have a check that got lost in the mail and you wrote that check and sent that check then you should call your bank or go to your bank and put a stop on that check.
There will usually be a fee to stop the check though so only put a stop on the check if you're sure the check has been lost in the mail.
If the check has been lost in the mail then put a stop on that check and then write out a new check and resend the new check and hopefully the person will get it.
If it's a check that someone sent you and it got lost in the mail then contact the person who wrote the check and they can also put a stop on the check and send you a new one.
You might also tell the post office but unless there was a tracking number on the envelope then there's not much they can do.