You should always attempt to get your Form W-2, Form W-2c, or
Form 1099-R (original or corrected) from your employer or payer before
contacting the IRS or filing Form 4852. If you don’t receive the missing
or corrected form from your employer or payer by the end of February,
you may call the IRS at 800-829-1040 for assistance. You must provide
your name, address (including ZIP code), phone number, social security
number, and dates of employment. You must also provide your
employer’s or payer’s name, address (including ZIP code), and phone
number. The IRS will contact your employer or payer and request the
missing form.
https://www.irs.gov/pub/irs-pdf/f4852.pdf
https://www.irs.gov/newsroom/what-if-my-employer-goes-out-of-business-or-into-bankruptcy
