Which plugin is this? I've just gone through this same problem: after months of use, Electrum stops opening, but it can be recreated from the seed if I move the electrum.dat file; so the problem seems to be related to the electrum.dat file. But upon recreation, I've lost all my notes, which is really a bummer since that certainly helped me keep track of what was what. a) Is there a way to get these notes back? and b) for next time, what is this manner of backing up the notes which you refer to?
Label Sync is the plugin to keep your labels encrypted in a web service to be able to have it synchronized between computers. It will avoid losing labels if the wallet file get corrupted, you can always use your seed to restore privkeys and addresses, but LabelSync is a good way to keep your labels backed up.