I keep getting the error: Photos was unable to open the library “Photos Library.photoslibrary”. (3142)

Where is your library now? Have you put it back from Trash into your Pictures folder?

If you already had started to empty the Trash with the library in the Trash, the library may be damaged beyond repair, and many photos missing.

If your Photos Library has been syncing with iCloud Photos you could create a new, empty library and then let your photos download from iCloud to the new library.

In that case try this:

• Hold down the options key ⌥ firmly while double clicking the Photos icon in the Dock to launch Photos.
• Release the options key ⌥ when the dialog to select a Library appears and select to create a new library in your Pictures folder.
• Now enable iCloud Photos in the Photos > Preferences or Settings > iCloud and wait for your Photos to come back from iCloud to your new library.

Once all photos have come back from iCloud you make want to make a copy of your Photos Library on an external drive, to have a backup of your photos. And enable Time Machine to make backups of your Mac. It is so easy to make mistakes and to lose our documents and data.