You can include Form 4852 in your tax return with the correct W-2 information. Normally you would first contact the IRS, as described at the link that Hal_Al posted, and they would send a letter to your employer. But since the IRS is essentially shut down right now, just go ahead and file with Form 4852.
When you enter your W-2 in TurboTax, enter the correct information instead of what's on the W-2 from your employer. After you click Continue on the W-2 entry screen, you will get a screen that says "Do any of these uncommon situations apply to this W-2?" (There might be some other questions about your W-2 information before you get to this screen.) One of the boxes says "Substitute form - I didn't get a W-2 from my employer and need to complete a substitute form." Put a check in that box (even though that's not exactly your situation) and click Continue. The next screen asks if you want to complete Form 4852. Click Yes. Proceed through the screens, entering the requested information to complete Form 4852. It will ask you to enter an explanation of how you calculated the amounts that you entered, and an explanation of what you did to try to get a correct W-2 from your employer.
