Low tech solution if it is a battery powered flash - turn it on
with fresh batteries, and leave it on at first without
discharging the flash immediately.  I would suggest 20 minutes
on standby, pop the flash once on manula, reduced power if there
is a way at first, then shut it off and repeat this for a few
days.  This is a low tech way to reform electrolytic capacitors.
I have a poweredful older line powerd studio flash, a Speedotron
2401. With it you can actually hear little arcs and snaps from
time to time for the first 20 minutes of conditioning, if the
flash has not been used for a while.  I am not super diligent,
but I do try to condition the studio flash pack every three
months if it has been otherwise idle. The power packed into the
big caps in thia unit can leave a a lot of havoc behind when
they blow. I have seen some fried examples at the repair depot
over time.