there is 3 possibilities:
1. you sent amount on the wrong adress!
2. you went to fish website!
3. your pc/mac is hacked!
It's definitely the 2 second option. He clicked on the phishing website which hackers and scammers send through slack and emails these days. It's easy to recognize the real myetherwallet but still people are careless. Sad to see your loss but it's gone.