My first reaction would be to clear off space on the hard drive corresponding to the scratch disk. Next, I'd try dumping the garbage, to free up any lingering space. After that, try rebooting the OS and seeing if PS will now launch. Have you tried all of that?
Press and hold down the cmd + opt (Mac) or ctrl + alt (win) keys while launching Photoshop. This will give a dialog that lets you specify a different scratch disk. (Make sure you have a second disk with available space mounted)