You most likely had a head speed too low, and coupled with the high angles that the blades go thru on a loop you just stalled the blades and kissed the dirt.

A loop without switching to idle up can be done, but you have to have:
1. enough headspeed to power the blades at the backside of the loop.
2. you will have to "stick bang" to create the loop, meaning that you have to have forward speed, leave the collective where it is and pull the elevator until you come around. NOT the best scenario.

You don't need too much negative on a loop, but engine RPM is what is going to save your chopper from kissing the dirt again. Just make sure you don't over speed the motor / engine while at zero degrees.

Rafael