it was a blocking diode or bypass diode. Jaycar have them. make sure you get one thats rated for at least 3x your systems delivery voltage (Nomimal panel voltageX # panels in series) AND at least double the number of amps you're capable of delivering (max panel amps X # panels in parallel)
if that doide was only a 20V diode and you put the panel it was in in a sting of 4 panels in series it could have been seeing up to 60v (based on 12V panels with an open voltage of near enough 20V) Similarly if it was rated for 10 amps and you had 4x8 amp panels fedding it it could have been trying to carry 24 amps.

