I don't understand how it would be corrupt. The file has been untouched since it was made. Seems like a quite a frequent issue with this software.
Unfortunately Prime does not provide any means for creating and maintaining backups. It also lacks a simple autosave-facility as we were used from real old Mathcad.
In the past we have seen many requests similar to yours because of damaged Prime files and I got the impression that it could have something to do with (lots of) embedded pictures (not sure, though). Sometimes parts of the file (which actually is a renamed zip file) could be restored, but sometimes we had also seen file which consists of nothing but a lot of zero bytes where nothing was there which could be recover.
But I haven't seen any such requests here recently. This may mean that users have given up, or (hopefully) that Prime's behavior has been significantly improved in this respect in one of the latest versions.
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