This type of situationssituation is all about initiative. If you contact them first, it looks like an honest error you caught. If they discover it and contact you first, then it looks like you tried to get away with something and they caught you.
Of course, in the latter case, you will protest that it was an honest mistake, but that just sounds like you're saying that because you don't have any other options now that you got caught. So you will have zero credibility.
So Right Now do whatever it takes to get caught on tape trying to tell them ASAP. If they are closed and don't have voice mail, I would write a note, put it in an envelope, and slide it under their door, making sure to get caught on CCTV doing so. That will put a timestamp on your efforts.
Unless this was a tax refund loan (which is often offered routinely to people who e-"file" with certain companies), your statements do have a credibility issue. One gets a loan for a reason, eagerly awaits the check, and applies the money to the reason. It's rather improbable to "forget". That, plus cashing it once with your bank and the second time through a check cashing place, raises some suspicion about your motives. All the more reason to take the initiative and get recorded with the earliest possible timestamp, trying to honestly rectify the situation.
Only as a standard disclaimer, I mention those online scams where "employers" "pay" you to cash checks into your personal bank accounts and send money on to them. We have many Q&A on that, read up if it applies to you.