Clean all the battery contacts with a pencil eraser. Use lithium batteries. I have had flash units I haven't cycled in a year, and they take forever to charge up the capacitor. Leave it on for a while, check it. Even if you don't get the "ready" light illuminated, try the "test" button in the hopes that you have enough of a charge for the flash to light. If you can get it to do this a few times, you should notice that it charges faster and faster. Hopefully back to normal.
I had one flash unit that I had stored for a couple of years. Clean contacts and new lithium batteries wouldn't get it to flash, even after waiting an hour. Luckily I had the AC wall adapter for that flash unit. Connecting it up to that power and it finally (after about 5 minutes) I got the "ready" light, and it flashed. I was patient, and did this several more times. and the capacitor started to accept a charge faster and faster. I was able to "revive" it and it works on batteries now. After this experience I cycle all my flash units every few months if they have not been used.
Good luck.