Corrupted Database File

A Test:

If you reconfigure your SCU and start with a Different PDB file.
The easiest way is to type in a PDB file name that does not exist ,  It will tell you is does not exist and ask if you want to create this file and use it.

Does Database Manager open and work correctly?  

If the environment is OK ,  then you have a corrupted PDB file and need a clean  / Not corrupted version of the file.  A Backup copy. 
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The system makes a copy of the previous database each time it is saved. You can look in your PDB folder for   FileName.pd^ 

You can also use the .FBK file for the project if you have created a back up that way.

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If you do not have a true back up of the database saved somewhere ,  then you need to start looking for CSV files where they exported the Database at some point.   You may be able to recover a large portion of the database by importing the CSV into a new empty database.