I had similar strange problem once.
Have you recently downloaded any "catorise"-ing apps ?? sometimes installing different versions at the same time could result in such problem and maybe you should remove it.

In case app manager has also stoped working-
Try to open app manager by opening a debian package on youe n900 -or- click a download link from the internet -I think- this should open app manager on your device.
If it works remove the package I mentioned above or any other app you have recently installed and which may has caused the problem.