Open up Access and create a blank database.
Goto File>Get external data>Import...
Locate the problem file and see if you can import the tables/reports etc.
If you can do so, then close the newly created database and see if that opens normally.
If it does then it could be the original file has become corrupted.
You might be able to repair it if you goto Tools>Database Utilities>Compact and repair... when you open Access on its own.
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