Your employer should have a copy of your W-2 form that they can send to you at your new address.
If all efforts to contact your employer have failed, you can enter a substitute W-2 form on your return using you information from your last paycheck for 2018. See the steps to do this in TurboTax below. Note that your tax return will have to be printed and mailed if using a substitute W-2 form on it.
https://ttlc.intuit.com/replies/5451995