Restore Photos Library

Can't you just move the library from the Trash bin back to the Pictures folder and open it?

Here's how you can recover a full library from your iCloud library:

1 - create a new library.

2 - designate the System Photo Library.

3 - enable iCloud Library in it.

4 - if you do't have enough room for a full sized library on your Mac the enable "Optimize Mac Storage" in the new library.

5 - and wait. Patience is a virtue and, in this case, a must.

It can take some time, over a week or so depending on the size of the library and the speed of your internet connection. Keep the library open and in the background during the day and at night so the recovery can proceed in the background,

As your photos are still in iCloud Photos and visible on your other devices, it will be much faster to start over with a new, empty Photos Library than dragging the library back from the Trash to the Pictures folder, if Photos does no longer recognize it as the system Photos Library.

• After dragging the library out of the Trash into the Pictures folder, double click the library to open it in Photos. If Photos can open the library, open the Photos > Settings, go to the General tab. Is "Use as System Photos Library still enabled? Then you are lucky and go directly to the iCloud Tab. Is "iCloud Photos" still enabled? Then just wait for the iCloud syncing to continue.
• But if Photos is no longer using this Library as the System Photos Library, you will have to click again "Use as System Photos Library", and only then you can enable iCloud Photos again and will be in for a very long wait, while all photos from the library will again be uploaded to iCloud and merged into your iCloud Photos Library. This will be a waste of time. In that case it would be faster simply to craete a new library, as OT suggested and make the new, empty library your system Library and iCloud Library. As the library is empty, there will be no new upload necessaty. The syncing will begin with the download from iCloud.

Click on the trash icon, find the library icon there, right click on that and select 'put back'

"Put Back" only works if there have been no other operations between going into the Trash bin and using Put Back. So you'll need to click and drag it to the Pictures folder.

The idea is that the Mac doesn't want to be continuously accessing iCloud and slowing down Photos while you're using the app, so Photos should be in the "background." That means it works when you use another app in the foreground (or just pretend to)-- open Mail and have it on top of Photos, for instance. Then you tell your MacBook not to go to sleep for the next few days:

Frankly, I don't think anyone really knows how much of this is really necessary, but why not?