Yeah, we need more stats to really make a good assessment of what went wrong, what's going on now, and how to possibly fix the situation. Break out your tests and start measuring! We need more info on the tang. Was this a recent addition?
Aside from that, I must caution you, many new hippo tangs often lay on their side, pressed against a rock in an attempt to hide. Tangs that are startled may also do this. If the tank is very.... er.... spartan (unaddorned), the hippo tang might just be trying to hide the only way it can at the moment! If you just added the tang and this seems the case (ie- it's still breathing, but has either wedged or pressed itself into a rock crack or against something), just try offering it some nice big holes to hide in and turning off your tank lights (or, at least, you day lights). That behavior is actually quite common in hippo tangs (often requiring LFSs to actually specially label tanks with new additions, saying "I'm not dead! I'm shy!")
However, if that does not seem to be the behavior, or, if there are any unusual marks on your tang, it could be something else. Try turning off the tank lights and testing the tank to see what's going on at the same time. At the very least, the dark will help your tang calm down a bit while you get a better look and a better idea of what's going on.