HP Bar disappeared in HUD but not on top of character

Playing Sona and in the early game my HP bar disappeared in the HUD. The bar would still flash red when I took damage and still displayed the number but was always black instead of green. Unfortunately I forgot to take a screen cap and I have no idea how to reproduce the bug. If it helps the 4 champs that were in the vicinity and attacking (besides minions) were Miss Fortune, Twitch (opposing team) and Graves (friendly).