several ways
1) contact the employer for duplicate
2) get a transcript online from the IRS. you need to create an account if you don't have one
https://sa.www4.irs.gov/secureaccess/ui/?TYPE=33554433&REALMOID=06-00008e4d-63f6-1187-a229-7c2b0ad00...
3) get it from social security. there is a fee
https://faq.ssa.gov/en-us/Topic/article/KA-02501
