My photo library is not showing up in Photos app

So is your library set as the system photo library? If so, and it's the only one, and it appears to be empty, then something has gone badly wrong.

Since you say you have all the photos in iCloud, then the simplest way to recover will be to sync iCloud with a new empty library, as follows.

1 - Create a new EMPTY library in your pictures folder. (Hold down the option key while starting photos, and click "create new". Name it something that is easy to recognise). If you don't have enough space on your mac to download the full iCloud library with plenty of headroom, then you'll need to put the new library on an external drive (formatted APFS or MacOS extended)**

2 - Open the new empty library in photos. (it should open after you click "create new" but if not, use the option key start again, and select the new library from the list.)

3 - In photos preferences set it to the system library (click the circled button)

4 - In photos preferences iCloud - turn iCloud on with download originals.

5 - Wait - for photos to download all your images from iCloud into your freshly minted library. It may take some time - possibly days. Leave the mac on with photos open and sleep disabled while it is doing it.

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** If using an external drive:

It must be formatted APFS or MacOS extended (journaled) (preferred for spinning drives). Not case sensitive. If you have not formatted it after purchase it will most likely be ExFat - which is not compatible.

It Must not be (or have been) used for time machine. (A time machine drive can be used if re-formatted)

It must have the 'ignore ownership" checkbox ticked.

It must be directly connected (No ethernet or wifi)

See

Move your Photos library to save space on your Mac – Apple Support (UK)

Hi

It sounds like you have somehow opened an empty library.

Hold down the option key while starting photos. More than one library listed? open each in turn until you find the one your pictures are in.

Start with the one marked system library.

I have the exact same problem. Tried everything suggested and no results. Updated to new Monterey 12.5.1 on M1 Mac Studio. Only the 55 People show up.

However when I opened the .photoslibrary file with another User all 13,000+ photos show up and back to normal. So it appears the corruption is something in the user specific part of the Photos app.

Any suggestions?