Well, I am guessing that you ran into one of the several monsters that can drain levels. They are VERY dangerous. You need to check the capabilities of the monsters you face before you start the battle and be sure to take out any which can 'Drain' very quickly. Note that monsters that can drain can only do so if they are the first monster in the list (i.e. the Drain option is a melee option for the monster and cannot be used from the back).
Also, if you have created a recent portal, you can always restore your game back to that state.