Flashes stored a long time often suffer from capacitor failure. The first thing to try however is leave the batteries in for a few minutes powered on to see if they will charge. If this doesn't work, then they'll need professional repair.

Moistures over the years do cause a lot of problems: check for sign of corrosion on the compartment battery terminals: if applicable, use cotton buds soaked in alcohol to remove all 'greenich/brownish' residues that may appear on the battery terminals and if heavy built-up scrap off with a screwdriver time as well.