Call your employer. You should have it by now. They were required to send it by Jan 31.
First the values on the paycheck and W-2 almost always differ somewhat and the IRS wants an exact match. This can delay any refund by weeks.
You also will not know what to enter for the various boxes on the W-2.
Employers have until Jan 31 to issue the W-2. If it is different you will probably have to amend which takes about 4 months to process.
If you mail file then you are required to attach a copy of the W-2 to the tax return or the IRS will return it to you unprocessed. While you are not required to submit W-2's with e-filed returns, you still must have the W-2 in your possession since the IRS can ask for it when they review your e-filed return.
If you do not receive your W-2 by February 23, then you can use your paycheck and file a 4852 form, but you must follow the IRS rules in order to do so.
[Paraphrased from IRS rules]
Can you file without having received your W-2? Yes, but only if you have not received it by February 23, and you have contacted your employer and attempted to obtain it, and you still have not received it by February 23, and then you contact the IRS (see the link below), then if the IRS is not successful in getting your W-2, you can use form 4852 in place of your W-2. But you MUST speak to the IRS before doing so. Otherwise, you cannot file before you receive your W-2.
https://www.irs.gov/uac/missing-form-w2-irs-can-help