Photo Library Lost on iMac

There are several keyboard shortcuts used with Photos.

• The key combination "option & cmd" should be used when starting Photos, if your Photos Library is damaged and needs repairing, for example, if you kept the library on an incompatible volume, or repeated crashes have left the library in an inconsistent state.
• Holding down the option key while launching Photos will bring up the Library Chooser dialog and let you switch between libraries.

They don't, your library should be right where you left it.

Try opening Photos with the option key held down. Is your Library shown in the dialog?

Glad I could help.