If you marked the task "complete," you can find the task and mark it "not started" again. On the other hand, if you deleted it, I don't have an answer...but someone else may.
Since I synch with a Palm, sometimes if I make that mistake, I change the task on my Palm, where it's not deleted, and then synch to recover the task I deleted on my laptop and want to get back. (or vice versa).