Will had some photos backed up to Google Photos for a few years, and recently, all the photos had disappeared. What happened? His current photos were uploaded, but anything before 2017 is gone. Leo says that's bad news because Leo always recommends Google Photos. It also proves that users shouldn't trust just one backup source for photos. Use several services. Also, check out the Google Photos trashcan to see if they are there. Leo says that if Will had used Picasa before 2017, Google may have gotten rid of them because they were stored in PicasaWeb. If he didn't move them into Google Photos, then when Google did away with Picasa, they went with them.
One thing he can try is to look at Google TakeOut. There may be some Picasa stuff there.
From the chatroom: ScooterX says he can still go to Picasaweb.google.com