Did you download the correct link for metamask? I'm thinking is that you might have downloaded the fake version of metamask and the seed that was given to you is also known by the creator of it. Which means that after seeing that you have sent ETH to it, they've sweep your balance.
lol READ what he said... omg
he MADE an ETH addy, SENT 12 ETH to it, then wrote down the seed words
NOW hes trying to restore it, but it wont take the seed... he says nothing about being robbed... now if he said "the ETH was transferred OUT", you might be right, but he didnt
@OP, theres no way for me to know from what u have said if i would be helping you jack someone (maybe you stole those seed words?) so i wont suggest how to solve this
imo, next time save a priv key

put it in a text file and encrypt that text file. problem solved
some time all these questions or answers just don't make any sense.