W/O being able to see what's going on, I say pay a pro to do the work. For now, of course, stay off the roof and out of the attic. If you need to do anything temporarily, get 2 2x4's an set them aside. Then get 2 pieces of 2x6s cut to about 48" in length. You want the temporary set-up to be such that the 2 48" 2x6s are the 'top' of your support system and the 2 2x4s are your posts. Place the support system such that the 2x6s span the adjoining rafters on each side of the one that's failed. Place the 2x4s vertically as if they're holding up the 2 adjoining rafters. Put 1 system just 'off' of one end that's broke and the 2nd system just off the other end that's broke. Basically, about a foot or so should be between the 2 systems.
One of the systems looks like this: --------- --------- | | | | | |
(If my drawing looks like crap, I'll e-mail you directly.)
Take another 48" piece of 2x6 to use as a base for each system whether you have carpeting or not. Measure the distance between the ceiling and the carpet/floor. Subtract the thickness of 3 2x6s (4.5"), then add 1". This value should be the length of your 2x4 supports. If you can't stand up the 2x4, cut off a little bit of the 2x4 at a time until you can stand it up. You don't need to wedge it up there so it's too tight; just enough so it's 'snug'. This should keep you until you get the rafter fixed. Now, once you've set up the 2 support systems, mark the 2x6s where you have the 2x4s. Take them down and nail everything together for each system. (Nail the top 2x6s to each other, nail the bottom 2x6 to the 2x4s, and nail the top 2x6s to the 2x4s as well. THEN, stand the system back up. Do the same for the 2nd system.