That's a "walkdown" - it happens to all the major rides a couple times each season. Usually what has happened is something is messed up with the computer and they can tell that it won't be fixed very quickly, so they want the riders to be taken off the ride, of course. It could also be a broken chain, burnt out lift-motor, or something to that effect as well. I've seen management give out the ride coupons, $5 worth of "Snoopy Bucks", and free small soda coupons, or give the riders the choice of which one they want.
Last year, my husband and I were waiting for Millennium Force first thing in the morning, and we waited about an hour (we were already in the queues, they'd walked us up partway), and the ride broke. So even though we weren't even on the ride yet, they gave us the $5. I thought it was really nice, since we weren't actually riding and everyone understands that these things happen.
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Po!nt of View: A different look at Roller Coasters.
http://www.crosswinds.net/~justmayntz/thrills/index.html