Sounds like the OS has lost the list of what filetypes Photoshop can open.
If only PSD is a problem, you can change it directly: http://www.bleepingcomputer.com/tutorials/reset-file-associations-on-your-mac/
If it's many types, you need to clear the LaunchServices database: http://superuser.com/questions/20822/where-does-mac-os-x-store-file-association-informatio n
Filetype is still set to the default of Photoshop. Only .psd files seem to be affected.