If you don't have a recent backup of the onboard data of your Zire 21, I'm afraid not much can be done. There may be programs similar to those used in a computer's hard disc to retrieve deleted information, but I have no info on them. There are MANY options to recover deleted data from other storage media, but from the inner memory of a Palm 31, I haven't found an option. The reason is that there are ways to undelete data from Palm Desktop, but that is not your current problem; and as for storage media in general, the Palm 31 doesn't support a storage format likable to undelete deleted records. I believe it is due to the nature of the VFS system.
"Do or do not. There is no try." - Yoda. "Nothing is neither wear-proof, nor fail-proof, least fool-proof." - HAL. "Indeed, fool-proof inventions have been attempted, but don't work, fools are pretty witty ones." - Murphy's Law. "Even worse than a traitor, is a dumb@$$ with initiative." - Gral. Santa Ana
Link: Palm resets