They climbed out, or died in the bin from over population or poor bin conditions are a couple of possibilities.

Here is a link that might be useful: My Earthworm Farm

Really we need a bit more info. Did you forget about it for a few months? did you harvest any castings? Did you feed them? Was it outside where it may have been too wet or the bin could have frozen or it could have gotten too hot? Does your bin have a lid or could some of the neighborhood varmints have eaten the worms?

Bryan

How many worms? Kept outside? What kind of worms? Feed them anything different lately? What kind of bin? Please give more info so we may be able to help.

Fito

Also, they may not actually be gone -- just hiding! I had a similar experience with my indoor bin; thought I'd killed them all. But turns out that they must have just been unhappy about something and were all balled up in a corner I didn't dig into, because a couple of weeks later, after adding some fresh bedding and stopping the feeding for awhile, suddenly the whole bin was crawling with worms of all sizes again. If your bin was outside, and it got cold, you may have lost some but I expect the rest are just tucked aways somewhere protected.

I don't really harvest the droppings but once a year, most the time I just use the bin as a convenient place to through food scraps. So I'm thinking that they probaly just got sick of wallowing around in their own fecal matter and hit the road. That is my current theory which I am deploying presently in an attempt to rectify the situation.
Thank you for your comments.