Also, the file was in .pln format. After I inserted the USB flash drive back into the computer, I saw .pln.t.s instead. Is it possible to fix and restore it somehow? (I tried to change the extension, it didn't work out)
try and use the repair feature that Archicad has as part of the open dialogue. If that doesn’t work I would try the backup file bpn and if that doesn’t work I would reach out to technical support and see if they can help you.
You don’t upload to the forum. If you have an ongoing agreement with Graphisoft then you will have access to technical support through your local Graphisoft office or Reseller. They may be able to help you.
If you do not have an agreement and access to technical support from Graphisoft there is probably no one here that can help you. As I said I would try and find a backup file bpn or another file of your project that may not be as current but one you can at least salvage something from.