Some time in the distant past, when I was still buying new gear, I read the owner's manual for a Vivitar unit [could have been a 283 or 285] that had a section devoted to reforming the unit's main capacitor after a long idle period. As I remember, the procedure was to load fresh batteries, switch the unit to manual and full power, power on, let the unit light its Ready lamp for thirty seconds before pushing the Test button, and repeat for a total of five cycles. The first charge cycle could take long enough for you to get real antsy, holding the unit close to your ear to be sure the voltage multiplier oscillator circuit was working, but by the fifth time around, the unit would be recycling in a much more reasonable time.
I just took delivery today of an 'AS-IS' Sunpak 611 that was marked 'won't power up' and ran through the procedure after running an emery board across the battery cover contacts. The unit needed a few more than five flash cycles before lighting up its 'FULL' indicator lamp, but it looks like it will be OK for use.
That 611 is a great flash. I've had mine since '76.