Do you happen to have a microSD card in the phone now? If not, do you have one that you can put in?
(I'm trying to avoid suggesting a factory data reset, which will wipe the phone of all data and apps, as long as possible... but I'm afraid that may be the ultimate solution.)
Based on your description, it appears that the internal storage location filled. I didn't think that this would cause a boot failure, but perhaps that is what is doing it. I don't know of a way to access it without the phone actually running, though.
It is also possible, though, that the error that you received in the camera app was a symptom that the flash storage in the whole phone has gone bad, and, in that case, even a factory data reset may not help.
Anyway, if you do have an SD card in the phone, try pulling it. If you do not, but you have an available card, try putting one in.