Well this is strange, I unfortunately cant tell you anything unless you have creative mode on :P if you dont, it could either be a problem with the computer or the mod itself, restart your computer, and if the problem persists, redownload the mod, thank you.

Really? I have a problem quite similar to this. Nothing can kill me, not even the /kill command. My health sats at mask and it's a game breaking bug.

Ahh, this often happens on servers and frequently on my own, I am afraid to do this you will either have to go to the files and delete your player.dat folder on your world, or delete the world save if you don't want to uninstall and re download the mod, on servers this can be fixed similarly by deleting the players.dat folder on the particular world.

I'm afraid you will have to do what Infinaminer said...

Also, which version of the mod are you running?

Well, for me, I just log out and back in. I don't know what to tell you after that.