REQUEST
Could the OP, or someone knowledgeable on this type of situation, speculate on the ACTUAL mechanism of this attack? What did the OP actually do, that gave away his private key? Was it stolen from the computer's clip board (copy/paste?)
Knowing HOW this kind of stuff happens will make it easier for other people to be more secure.
Thanks
It is impossible to know since the OP does not give too much information, the most likely thing that happened is that he had installed the electrum wallet in an insecure OS like windows and then he download some pirated software or something that infected his computer with a key logger and then when something interested was typed the key logger called back home to alert the hacker and he stole all the money.