"First, never ever try to attach (or detach) a lens to a body when both of them aren't cocked. That's a fast way to do about $250 bucks worth of damage. Sometimes the lens shutter trips and closes by itself from one thing or another. To reset the lens if it's off the body, take a coin insert it in the rear reset-slot next to the red pin and turn it clockwise until you see the shutter open and lock into place. A flat blade screwdriver works for this as well but be careful not to slip as to not scratch the rear element.
If the lens is on the camera and the body is cocked but the lens shutter isn't, take a long screwdriver, push open the rear shutter curtain, find the reset slot and again, rotate it clockwise until it locks into place using the same precautions I mentioned.
If the shutter is tripped on the camera, just advance the film advance knob or crank to reset it but always make sure that the camera is cocked before adding or removing a lens.
Another thing is never try to attach or remove a film mag without having a dark slide inserted in the mag. It can be done without the dark slide but it'll damage either the mag, the body or both.
AND, if you suspect your film mag(s) haven't been serviced in quite awhile, either replace the light traps yourself or have a repair shop do it and while they're in there, clean and lube and adjust the gearing. Light traps are easy to do yourself with a jewelers screwdriver and you can buy the traps for about $10 bucks each from a guy on e-bay. Just a piece of foil basically and a chunk of foam rubber.
Also, if you buy any additional mags, make sure the 3 digit number on the insert matches the last 3 SN digits of the magazine. That will tell you that the insert was factory matched to the mag. If some guy tries to bullshit you and tell you it's not important, walk away from the deal.
Never ever try to adjust the focusing screen by yourself. The 500C is not like the CM or newer models that allow you the luxury of changing or aligning screens without sending them to a shop for service. To me, that's the biggest drawback of the "C" bods. Another thing to watch for is tripod insert/adapter thread stripping out and pulling loose of the body while it's mounted on a tripod. The cheaper inserts strip and don't fit properly. OR, someone at some time forced an insert in to the bod, stripped it out and voila, the camera body thread needs to be retapped. Easy to fix, potentially disasterous if you don't notice. That's really about it. Great camera. Yikes, I own four of these things and use them nearly every day. So if you ever have questions, feel free to drop me a note and I'll be glad to help you out. The "Hasselblad Manual" by Ernst Wildi is very useful and well-written. Available used at Powells.com even the first or second editions will do when you have a C or CM.
Also, if you ever need repairs, THE guy is David Odess. http://www.david-odess.com/index.html
He trained at Blad and has been doing repairs for nearly as long as I've been shooting. [That's about the time Moses was in short pants].
It's a wonderful piece of equipment. Use it in the best of health !!!!"