The same thing happened to me when I was installing all the post jailbreak files. (I had just jailbroken a kindle keyboard and a dx so I thought I should install the same extensions that were recommended for them.) I don't remember if I ended up doing a factory reset afterwards or if a hard reboot was enough to get to the point of being able to fix it.
I posted a
request in the thread with NiLuJe's snapshots to add a note about this in the extensions section. Hopefully this won't happen to too many others.