Have they refunded you already or is it still pending in your account?
First thing is to contact the restaurant directly by phone, not just wait for replies through the app. Explain that you have a reservation confirmation and that you paid. If they say it was a technical error, ask them to guarantee you the table or refund you on the spot. A lot of the time this gets sorted quickly when you talk to them directly, because they have some room to fix it if things get serious.
If the restaurant is being difficult or won't respond, gather all your documentation (screenshot of the reservation, payment confirmation) and contact TheFork directly. They have an obligation to mediate because you did your part: you paid and you have a valid confirmation. The platform usually sides with the customer in these cases, especially when there's a "technical problem" involved that's not your fault.
Either way, if this doesn't get sorted by Friday, think about booking at another restaurant in the area. Sometimes it's faster than waiting for them to fix the mess, and at least you'll have guaranteed you can eat out peacefully that day.