The razor head comes loose from the two arms.
I had a similar problem where one of the side covers popped off. I was able to reattach it, by carefully aligning the spring and pushing back into place. If you've lost the pin, you're out of luck. Braun does not make these kinds of parts available for user servicing, unfortunately.
However, the 7000 series models are all compatible with each other in terms of motor, battery, and razor head armature. You could try locating a moderately used example on the cheap (maybe less than $20) and then cannibalize it for the parts you need.
Do NOT send it off to Braun for servicing, as you will be charged much more than the shaver is worth. If it comes to that, you're better off just going with an 8000 series shaver.