One - click the Switch/create tool in the Library icon drop down
Two - that will pop this dialogue box
Three - click the blue folder button and browse to folder in which you found the metadata.db file - don't open the folder - select the folder and click Select Folder. That will take you back to the calibre popup dialogue box.
Before clicking OK in the calibre popup make sure the "Use a previously existing library…" radio button is selected.
You should be able to switch to that library.
Where is the library?
BR