So I just got an SX-70. I didn't know if the seller sent to me an empty cartridge, so I opened the film door to see it. I took it off and inserted it again... The cartridge is old and it seems that it doesn't have enough battery by the slow noises that the camera made. Now I think the mirror is stuck (I only can see half of the "picture" through the viewfinder), the film is stuck in the rollers, and the problem is that I can't open the film door. What should I do?