I'm sorry you fell victim to this scam, but unfortunately, it is a pretty widespread internet scam. Lots of companies run similar schemes. Think about it--how could they have "approved' your loan if they didn't already have your information?
The main idea is to walk away with that admin fee. But if they also have your personal info, like your Social Security number and maybe the name of your bank, they may also be engaged in Identity Theft. They may try to empty your bank account, charge stuff to your credit cards, or even apply for loans from other lenders using your ID.
You need to tell *all* of your banks, credit cards, etc. to watch for suspicious activity. And you should also contact the Attorney General's office for your state. I doubt that they can get your money back, but it's worth a shot.
Again, I am so sorry to hear that you've become a victim. These thieves count on people who have a trusting nature. But now you need to act fast. Put a hold on every account you can. Banks and card issuers will understand your situation. Unfortunately, it is far too common. I wish you well.