Typically reloading the game will fix this, as your browser (The client) believes one thing while the server believes a different thing. Since they don't agree they are fighting each other.
With some browsers and situations you need to clear your browser cache, close, and restart your browser to get rid of the bad data.
And 0% is not necessarily dead. It could conceivably have 0.01 hit points which rounds down to zero.
If you continue to have this issue after clearing your cache send a bug report from within the game and we can refresh the server from the database to clear up discrepancies.
