I got a w2 from my former employer, claiming they paid me a "x" amount but I never got that money, what can I do?

Since you already filed, you will want to try to find any proof of what you were paid (bank statements with direct deposits or when you deposited checks) and any paycheck statements that you were given. 

You should call the IRS at 800-829-1040 for assistance in March, and explain that you already filed without the Form 4852. You must provide your name, address (including ZIP code), phone number, social security number, and dates of employment. You must also provide your employer’s or payer’s name, address (including ZIP code), and phone number. 

You may also want to contact your state Department of Labor (State Labor Offices) and file a complaint- they can investigate and put a claim in to see if they can recover some of the wages you did not receive.