Why wont my app open!

I have no idea, I don't even now what app it is.

Many of the reasons why applications won't open are also the same reasons system slow down. See if any of these apply: https://discussions.apple.com/docs/DOC-6921 And yes, by all means give us more details. Go to Apple menu -> About This Mac and tell us your Mac OS X version. Go to the application in the Finder and select it by clicking it once, and Get Info from the File menu and find out the version of the application. Then compare to http://www.roaringapps.com/ for compatibility data. While in the Finder find your free space and space used in the Finder status bar (activated by the View menu when a Finder window is open).