A little off topic, but, a couple years ago I had a bulb once where the outer globe somehow snapped off (right in two, the break point right above the inner arc tube). I happened to be home for lunch that day, the light had been on about 1.5 hours when I noticed something was amiss ... I noticed something was amiss because my acro that was right under it was totally bleached, apparently by the UV burn. That coral didn't make it
An interesting side note though, where the glass had broken, and the end part was sort of leaning off (didn't fall completely off because of the wirefram inside the bulb was supporting it mostly), a piece of it touched the arc tube (that was still lit). Where it touched, it had melted. Wow, that's hot.
Anyways, I'm not sure I'd run a damaged bulb. If nothing else, there is the UV risk if it's the outer envelope that's damaged. And if it was the inner envelope, the arc tube or whatever it's called, that was cracked I'd have to wonder, if nothing else, what sort of spectrum I was getting and what sorts of elements might have leaked out, or whatever. I don't understand the science of bulbs.
Andrew, you're screwed, you need a new bulb. Sorry. But there you have it. Unfortunately, s--- happens every so often. Your corals will be fine for a couple of days without light, but the longer they are without light, the more you will need to back off your photoperiod and re-acclimate them to the light once it's replaced, otherwise you run the risk of burning them.
If I was you, I'd probably jump on Kari's offer ... and then saving my pennies for a new bulb. Yeah ... I know it sucks ... I've been there. Nothing worse than having an unplanned big purchase, but, unless you have a backup bulb (hanging onto your old bulbs for this purpose, once you've replaced them, is probably the sage thing to do), there are not a lot of options here.