If you generated your seed on this website, you fell victim to a scammer.
Online seed generators in general can not be trusted and should never be used to create a seed.
What they do is they pretend to generate random and unique seeds, but in fact they are not.
They generate only a small range of pre-determined seeds, which are all known already to the owner of the website.
All he has to do is to check these seeds for any incoming funds and take them whenever he wants to.
The moment you made your seed on that website, the thief already had it.
You are one of many victims of this criminal. It started in January and apparently it's happening until today.
You can read more about it here: https://medium.com/iota-demystified/what-happened-last-night-on-iota-b6157ade1e03
It's important to understand that this has nothing to do with IOTA or the wallet.
If you create your bank account log-in name and password on a malicious website, your money will get stolen too.
The only safe methods to create a seed are all in this blogpost: https://blog.iota.org/the-secret-to-security-is-secrecy-d32b5b7f25ef
Once Trinity wallet comes out of Beta, you can of course also use the integrated seed generator. I think it is secure already, but Beta is still testmode.
Unfortunately, there's nothing you can do now, except file a claim with your local (cybercrime) police and hope that the thief gets caught one day.
If the thief ever gets caught, you will have to provide proof that those stolen coins belong to you.
So even if you don't want to involve the police, please make sure to keep your seed and all evidence of where your bought your coins from and where you sent them to afterwards (for example save your trade history and your withdrawal history from your Bitfinex account or wherever you purchased your coins).
I'm sorry that you've lost your funds!
I wouldn't be able to help anyone without thetangle.org-explorer. If you feel like you want to support its developer Mathieu Viossat in maintaining his service, please consider a donation to the address shown here: https://thetangle.org/about. Thank you!