Photo viewer. . .
Please help us out a bit. From your message, it's hard to tell if Photo Viewer has gone missing or if what you're experiencing is a side-effect of some other issue.
Locate a picture file in Windows Explorer and Right Click on it. Do you see the option to open the file using Photo Viewer?
Create a new Shortcut for Photo Viewer. Right click on an empty spot on your Desktop and select NEW. When your system opens a NEW box, select Shortcut
In the Location box, enter
C:\Windows\system32\rundll32.exe "%ProgramFiles%\Windows Photo Viewer\PhotoViewer.dll", ImageView_Fullscreen
Click Next and give it a name. (You likely don't want to use the default "RunDLL32")
See if either of these things work.
Cheers, PW.
P.S. The shortcut you create will open Photo Viewer but the program won't do much. To get it to do something useful, you must also specify a folder containing image files, for example. . .
C:\Windows\system32\rundll32.exe "%ProgramFiles%\Windows Photo Viewer\PhotoViewer.dll", ImageView_Fullscreen R:\
This shortcut string will open image files in R:\