In place operating system upgrades are a crapshoot and to add to that CS5 is not supported under Yosemite.
You can try uninstalling and running the cleaner tool then reinstalling. If that doesn’t work, try a clean install of the operating system.
No matter, if none of this works, I’m afraid you’re on your own.
Sorry to say it, but I think you can see from Bob's response that there's no way to solve this rapidly.
Did you take an image of your Mountain Lion install before upgrading to Yosemite? Roll back to that. If you're not in the habit of taking a snapshot of your system before performing operating system upgrades - get in that habit. If you are dependent on anything at all on hour computer, having a reliable backup method in place is essential. If you don't have a Time Capsule, or some other way to run Time Machine onto a disk that's not in your computer, go set that up yesterday.
If you can't just remove Yosemite and roll back to Mountain Lion for whatever reason, you can partition your drive so that you can install both Yosemite and Mountain Lion on the same drive, and then boot into Mountain Lion when you need to work in CS5. Or you can take your Mountain Lion disc (I assume you have one, no?) and then use it to create a virtual machine in something like VirtualBox to run Mountain Lion from inside Yosemite.
IIRC, there were no Mountain Lion disks. It was download only from the app store.