In my experience, USCIS did not accept the reference/experience letter for I-140 from my ex-collegue who is working for a different employer now. The letter was on his current employer letterhead.
If the company doesn't exist, there are couple of options, you could execute an affidavit stating that efforts were made to contact the employer and you could not reach him. Also, you need to find an another employee from the same company who can support your argument.