This sounds like a problem with your preferences. First try this.
1. open a new scene and create a sphere. Can you modify this? If yes, open your other scene and select anything that your want to keep (not default cameras). Do your selection from the outliner. File > Export Selected. Now open your newly exported scene.
2. If that did not work, you need to toss out your preferences. First shut down Maya. The preferences are located (this is a windows 2k path if Maya is installed on C drive) C:Documents and Settingsuser namePersonalmaya4.5prefs (where user name is probably your name)
you can either delete this entire folder, or rename it in order to save a copy. You may have things in here that you want to keep. Your shelves, icons, marking menus and hot keys are in here. The files that are generally corrupted are in the prefs directory level, not the lower directories. I would start with deleting the "windows prefs.mel" file and then restart Maya. Maya will, upon restart, recreate this file if it does not exist. If that does not fix it, delete more of the prefs untill Maya returns to working properly.
alan