Q: Photos will not open thumbnails.

What happens when you try?  Do you get any sort of message or warning?  If so what is it?

What viewing mode/window are you trying this in:  an album, All Photos, PHotos?

What has changed since it worked? Have your done any disk clean up or run any clean up software?

LN

Try the following:

1 - go to your User/Home/Library/Containers folder and move the folder named com.apple.Photos to the Desktop.

2 - go to your User/Home/Library/Preferences folder and move the com.apple.Photos.plist file to the Desktop.

3 - reboot and try Photos again from your account.

NOTE: In Lion, Mountain Lion, Mavericks, Yosemite and El Capitan the Home/Library folder is now invisible. To make it permanently visible enter the following in the Terminal application window: chflags nohidden ~/Library and press the Return key - 10.7: Un-hide the User Library folder.

 

For Mavericks, Yosemite amd El Capitan you can go to your Home folder and use the View ➙ Show View Options menu to bring up this window:

Are you in the Home folder where these folders are located?

Try repairing the library according to the instructions in this Apple document: Repair Your Library.

Warning: If you use iCloud Photo Library with Photos, this action will cause Photos to upload all your pictures again (from Fix missing or incorrect thumbnails in Photos - Apple Support).