You have a couple of options - none are easy, but can be used.
1) Contact your employer, request another copy.
2) If you have your last paycheck stub for the year, and year-to-date amounts are printed there, use those while preparing the return. Ask your former employer for the company's EIN (employer identification number.)
3) At irs.gov, create your online account, and review your Wage & Income transcript for the tax-year in question. Assuming the employer filed the required reporting, the report will include information that appeared on the W-2 which the employer was required to provide. (The employer name and EIN are however abbreviated to reduce the chance that this information could be used for fraudulent filing of a tax return in your name.)
