I am assuming you have the threat meter enabled..
ALT+O-press Advance (top right corner)-user Interface-Game windows
make sure you have always show threat window ticked.
If that still doesn't work
Press F10 make sure you are incombat and move your windows around as it may be behind them also make sure it isn't outside of the window same section as the threat meter just tick and un-tick that should pull it back if its off screen.
Failing that try a stand alone threat meter if that works then you may need to re-load profit .
