Hand held dopplers are notoriously unreliable, they are very user dependent and not as specific as a big ultrasound machine. I've had a few midwives unable to find the heartbeat, then another would come along and find it no probs. It's easy to say don't worry, but it's only natural to worry. You could always go to the GP, explain what happened and ask for a fetal well being scan. When is your midwife due back? Can she come and try again. That is horrible to leave you worrying like that.