|Model number is always useful to post...|
Quite a few Hitachi models appear to have problems with their psu - which seems to be in several sections/parts.
A google trawl brings them up of course.
But until we know the model number not much chance of anyone here being able to advise further.
However,unless you're into fault finding, and very likely component level soldering, you may need to take the plasma to a qualified techie?