Take a look at this:
1.Re: Photoshop CS5.1 not working with OSX 10.10 Yosemite
Please try and reinstall CS5: http://feedback.photoshop.com/photoshop_family/topics/problems-with-photoshop-cs5-after-in stalling-yosemite-version-10-10-on-imac It appears that the OS updater is corrupting PS CS5 if it was installed prior to updating to Yosemite. Let me know if that fixes it for you.
Courtesy of Jeffrey Tranberry
They are using CS5, but it just may be your problem too