How to obtain past work history if you have lost it?
Contact your previous employers by phone or email. They're required to maintain employment records for so many years, even after you leave their employ so it shouldn't be a problem.
In some states, when a request for something like that is submitted, it's illegal to with hold the information.
OR you can review your tax filings from the past that should have most of the information you're after as well (on your W2's, etc) and you're supposed to keep those records for about three years, or 5, depending on the circumstances.
go to the social security website and click on employment history. you will be charged a fee to obtain employment records of up to 7 years.