I had that once. I fixed it by doing the following:
- i used Maemo-flasher to disable the watchdog functions
- i booted the N900 with watchdogs off,
- i pressed [cntrl][shift][X] to open Xterm,
- i replaced the broken transitions.ini with a backup (hope you have one...)
- re-enabled the watchdogs with maemo-flasher
and now it works again.
For more info on what to do, and how to do it look here
link more precisely MohammedAG's post. For info on Maemo-flasher check the maemo wiki.
But be VERY carefull, and make sure know what you are doing!
Good luck!