Most likely a virus.

If you didn't have a password set, then a JSON rpc call can be made to get your seed and send a payment from a website so maybe that caused an issue somewhere...?

Without any more information on where it is and your general browsing behaviour we can't really give you much...

Sorry to hear that.
Was it due to a phishing email ?
Did you have some form of 2 factor authentication active on Poloniex ?

This is the downside to leaving funds on an exchange to daytrade, it makes your funds vulnerable.
Best is to just go in and out of an exchange as fast as possible.

its noe defeat. I will call it New Begining. Maybe we will see a little low and up maybe now. But hope for best