It would help to know details. Depending on location, $50 could be a resort fee or a charge for parking (although in many places tax would be added for parking, just like it would be for minibar charges, so it wouldn't be a round number for the total). Did you eat in the restaurant and perhaps tip to a round total? It might be a legitimate charge that was forgotten when you checked out, but usually that's just billed to your credit card. If you have a receipt, the receipt should tell you something about why they're charging you. Have you checked the rate for your reservation versus the rate you were charged?

Could it be an early or late check out fee? Can we assume that you didn't cause minor damage or take something worth about $25? [$50 is too little for a bathrobe and too much for a towel. Alarm clock?]

Try to call on a weekday and ask for their accounting department or the assistant manager on duty. Start by asking them what the charge is. I wouldn't just ignore it.

This seems so odd that no one is able or willing to tell you why you should pay an additional fifty dollars. Are you sure you're dealing with the hotel (and not perhaps some rogue employee) rather than some scammer? As I said earlier, the round number seems to eliminate some possibilities due to tax.In what form do they want you to communicate a new credit card number? Email? Phone?

BTW, if your credit card number was compromised, how did the timing of that event compare to your stay at this hotel? Could someone have used your credit card directly (not a room charge to you) to make a charge at the hotel during or shortly after your stay? In such a case, if the charge later was denied, the hotel could decide to contact you......or even if something like this happened with a Simth and your name is Smith, someone at the hotel could have decided to try charging the thing to you.

Even if there's no GM, the property surely has an acting GM, although that person might not know anything about the sort of issue you're experiencing.

I'd make sure to have an email trail that you asked for a copy of the original bill and you asked for an explanation of the additional $50 charge.

If you have a new credit card number that's essentially your old account (same bank, same affiliate card, etc.), at some point you probably should bring the credit card issuer into the mess. They might react negatively to a merchant pressuring you to pay a late charge that they cannot explain or document. IANAL, but at some point threats become extortion. I'd also watch the deadline for disputing the original amount on the ground that the merchant has consistently failed to provide an itemized bill for charges that they posted to your account upon checkout.

If everything is legitimate, the hotel simply sends you a copy of your original itemized bill (which should exactly match what was previously charged to your credit card) and a new itemized bill identifying the new charge. This should be easy, there shouldn't be a language issue, and again IANAL, but I'm not sure whether one would be obligated to pay some additional mystery $50 fee without any itemized bill or explanation. If so, any hotel could arbitrarily charge anything to any guest long after checkout. [It happens that hotels post charges to credit cards after posting the originally billed amount at checkout, but you should be able to get a detailed bill showing the new charges and/or dispute them with the credit card, although I suspect that often people just don't notice those erroneous minibar etc. charges and unfortunately hotels don't proactively send a copy of the new bill, which IMO should be illegal.]