I tried downloading iPhoto Library Manager but it says the data can't be located. Help!
iPhoto Library Manager can only repair iPhoto Libraries, not Photos Libraries. Or are you still using iPhoto and not Photos?
My very large photo library is all there in iCloud but on my Macbook Pro it can't find Photos.
Does that mean you have uploaded your Photos Library from one of your devices to iCloud by syncing with iCloud Photos? Then Photos for Mac on your Mac should be able to download the originals of your Photos, if you enable iCloud Photos in the System Preferences > iCloud > Photos and in Photos > Preferences > iCloud. It will not work, however, if you are using more storage in iCloud than you signed up for, or if the file format of the image files is not compatible with the system on your Mac.
- Are you perhaps running out of storage in iCloud or on your Mac? (Manage your iCloud storage - Apple Support)
- What is the file format of your iCloud photos? Have the photos been taken with an iPhone with a relatively new system? iOS 11 or 12? Then the format of the photos may not be readable on a Mac with an old system like OS X 10.11. If the file format of the photos is HEIC, you will need to upgrade the Mac to macOS 10.13 or later.