"Unable to Load" happens when the Photos database has lost contact with the "original" picture file that was imported. One possibility is that the original file was not actually loaded into Photos' Library package on a Mac at all. In Mac Photos' Settings>General Menu

"Copy Items" should be checked. If at any time that was un-checked, then Photos relied on finding the originals in a specific location in the Mac's Finder, and if that is changed without notifying Photos, the connection will be lost. If you can find those originals, then sometimes the connection can be re-established. "Copy Items" is the default setting, and it really shouldn't be changed.
Of course, various kinds of corruptions can cause the same sort of problem. For instance, have you had your Mac's Photos Library on an external drive? If so how was the external drive formatted? To avoid damaging the Photos Library an external drive must be formatted in either APFS format or Mac OS Extended (Journaled) format. The drive must be connected directly to the Mac by cable, not networked, clouded, NASed, etc. Additionally, the drive can not have had Time Machine on it since it was formatted. There have been so many problems with using incompatible drives that the newest macOSs won't even allow a Library on a non-Mac formatted drive to open, since there is a chance of damaging the Photos database. See this:
Move your Photos library to save space on your Mac - Apple Support
If the connection to the original is lost, the thumbnails and preview versions, with less resolution or quality, may still be available.
How large is your Library?