Photos on Yosemite 10.10.5 just stopped working

With the amount of information you've provided it's impossible to offer a solution. We can't see your computer so we need to know the details of your problem and setup, i.e.:

• what system version are you running?
• what version of Photos are you using? Control (right) - click on the application and select Get Info from the contextual menu. The version number will be in the Info pane.
• how much free space on your boot drive?
• how big is your library, i.e. how many photos?
• where is your library located? If on an external HD how is that HD formatted and connected to your Mac?
• did Photos work previously to this problem?
• did you apply any updates or upgrades just prior to the problem occurring?
• are you running a "managed" or "referenced" library?

• have you run any "cleaning" or "optimizing" apps on your Mac?
• what fixes have you tried?
• launch Photos with the Option key held down and see if there are more than one library listed in the Open window.

Go to your Users/Home/Library/Containers folder and move the com.apple.Photos folder to the Desktop.

Reboot and launch Photos again. See if you can get into its iCloud preference pane then.

Tell us exactly what your Photos preferences ==> iCLoud are and the status - post a screen shot if you want

If I understand you correctly this started when you clicked some iCloud service - until we have the details I suspect that time will resolve this and it is just an initial set up issue

LN

what has changed since it worked? Have you "cleaned" you Mac in any way?

LN