Push it firmly into the off position before you try to reset it. If that doesn't do it, it may have flaked out, though that's fairly uncommon. What brand and type is your panel?

Try pressing the breaker switch all the way over to the OFF position. Then switch it to the ON position.
If this doesn't work, the breaker is probably broke.

OK - it is all the way over to the OFF position. It doesn't seem to be one that half-trips, though I can't figure out how it DOES trip, now that I look at it.

It's a Craus-Hinds I think, and I'm not sure what you mean my type? It's actual circuit breakers, not fuses, 100 amp service.

Thanks
MPEDrummer

Unfortunately, you now have another problem. Specifically, finding a breaker that fits a Crouse-Hinds panel. (That division of Crouse-Hinds no longer exists.)
This forum has had several discussions on substitute breakers for Crouse-Hinds panels in the past.
Your panel may have a sticker that lists approved substitues or you may find a Westinghouse, Bryant, or other breaker (series BR, I believe) that will fit.
Bay Breakers and Pacific Coast Breaker advertise having a stock of Crouse-Hinds breakers.

Wow...I didn't realize just how screwed I was.

My electrician finally called back - he's telling me that I'll need a whole new service. Do you think that's the case, or is it possible to find a new breaker for this panel? I'm going to go try to find the model numbers, see what I can find out...I really can't afford this right now, in so many ways.

What a lousy day.

MPEDrummer

Bay Breakers, Richmond, CA, lists a reconditioned model MP2100 for about $50. You can call them with your model number at 1-800-699-2980. (I've never used them, just found them on the web.)
Your electrician probably doesn't want to mess with used parts and knows that inspectors often don't take kindly to substituting breakes from other manufacturers.

Here is a link that might be useful: Bay Breakers

You guys rock.

Bay Breakers is overnighting it to me, I should have an exact replacement in hand tomorrow.

Thanks!

MPEDrummer

With enough patience, flipping it on and off over and over some breakers will eventually reset. This is only for breakers that you know are just being lazy and not for breakers tripping from a short or ground fault. Replacement is best but when you are really stuck and need the power back now try dozens of cycles before you give up.

I had the same issue. I discovered, by trial and error, that if I turn one of the low amperage breakers on (15-20 amps) that would hold the main breaker on. With all of the breakers off the main breaker would not stay on, even after repeated tries. Hope that works for you too.