Heat pump - outside unit fan not running
Sounds like your defrost cycle isn't kicking in.  There is a defrost circuit in most newer units that controls that cycle, in which the unit runs in reverse -- literally taking heat from your house to defrost the unit.  I'd turn it off ASAP if that's the case since it's just wasting electricity and call a tech to have a look and possibly replace the circuit board.  Tell them what's going on ahead of time and maybe they will have the part with them when they come out!