With a little luck, you may just need to replace both your belts, and possibly a belt tensioner and/or pulley, and you should be fine, IF the car did not overheat for very long.
You say the belt came off INTACT? That is unusual, but if one of your belt tensioners failed, then that could explain it -- replacing the tensioner is not a big deal (the part is about $100-150 I am guessing and just bolts in). If you don't know already, there is a tensioner for each belt, and each has a free-spinning pulley on it for the belt. It keeps the belt at proper tension and keeps it from flying off its pulleys. The power steering is driven by a belt, so that should come back when the problem belts, tensioner and/or pulleys are replaced. I think you probably have mechanical (spring-loaded) tensioners like I do.
Alternatively, any one of the pulleys on the affected belt may have completely failed (fallen apart). Very easy and inexpensive to replace pulleys.
If you want a visual, your tensioner and pulley for the small belt can be seen in the bottom diagram of this webpage, and the tensioner for the large belt can be seen in the diagram just above it. They are the "number 2" part in each diagram. Click on the diagram.
http://www.realoem.com/bmw/partgrp.do?model=AM33&mospid=47718&hg=11&fg=18
If the car overheated for a longer time, cracks can develop in the aluminum head, and that is some very serious trouble. If that happens, you would get either oil in your coolant (look for chocolate-looking mess in your coolant) or coolant in your oil (which often will show up as a lot of white exhaust "smoke", and/or chocolate-looking oil), or you can get leaks in both directions. Some people have a mechanic install a "good used" engine from a wreck instead of trying to replace the head. Both probably cost approx the same. Hope you don't have any of these problems. If your temp gauge ever goes into the red, stop your car immediately and drive no further at that temperature.