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I have 2 550Ex's as well as lots of 430EX's, I have never had one fail like that just through lack of use.
Over time the capacitor can get deeply discharged, I wouldn't necessarily give up, try some different batteries.
Also use a q-tip and some solvent to clean the battery contacts at both ends, they often get corroded and prevent the gun firing up.
I also have five Vivitar 283's that are very sluggish to start taking a charge after a long time off, but they have always come back in the end.