Two possibilities come to mind.
Either you simply knocked the video connector loose, and it's a relatively simple fix, or you did permanent damage and unless the system has sentimental value, won't be economical to repair.
Check the ifixit website for instructions on taking the iBook G4 apart to get to the video cable. If that doesn't fix it, start looking for a new laptop.
Computer was dropped. Now when turned on, the computer chimes, you hear the HD etc but the screen simply alternates blue, green, red and grey. Any ideas?