Do you use PitBull? If yes look in the HideBlizzard module for I think it's called Alternative or Blizzard Powerbar and uncheck that and it should show again, or enable the alt. Powerbar on your unitframes(should be under Indenticators).
There are probably some other UF mods that block that aswell but PitBulls the only one where I know how to get rid of the problem.
Only other thing I can say for sure is that it isn't StUF if you happen to use those.
You can either post a full list of your addons here and wait for someone to say "Hey that's easy disable XXX" or you can disable half of your addons see if it's still blocked. If not disable half of the remaining addons and so on until you found your culprit.
Hope I could help.