Companies are closed

[spna]

I have foreign work experience of about 3 years. On the beginning of my career, I worked at 2 startup companies which are now closed. So I can't get a detailed reference letter for about 2 years of my experience. I have an offer and experience letter from those companies but it does not have job duties or working hours listed. What other document can I submit to prove my experience? Do I need an explanation letter. If yes, What details should I include in explanation letter?