Filename.mdf is not a primary database file!

Dear friends, need your kind and quick assistance here.

I'm working as an SA in my company and handle my SQL Server. One of the important MDF files is not valid as my users are getting this error: "Filename.mdf is not a primary database file".
Is this corruption or a minor issue? From where should I start to fix it?

Please help me. :pray:

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