Where does this problem come from?
Because of the limitations of an Access database, it sometimes happen that you can not open your planning anymore.
How to solve this problem without sending our support your database?
Here is the process to follow in order to solve this problem immediately:
Open the "pp" file with MS Access.
You will then get this message:
Then, close MS Access and open your database again. The planning is now working.
How to avoid this problem?
Either you decide to migrate your database from MS Access to SQL Server for example.
Or, you can purge your planning (Menu File -> Database -> Purge) and compact it (Menu File -> Database -> Compact) in order to reduce the size of the database.