To replace your lost, stolen or mutilated EAD, you will have to file Form I-765, Application for Employment Authorization. Tick the box “Replacement (of lost employment authorization document)” on the very first page of Form I-765, below the words “start here”.
Employment Authorization Document is a physical proof of your right to work in the USA. For some people it is also the only government-issued identification document. So, it is advisable to replace your work permit as soon as possible.
A new EAD will be issued to you, if you still are an eligible applicant, approximately within 3 months. Meanwhile, while expecting your replaced work permit, in some cases you may use a photocopy of your lost/stolen work permit or the I-797 Notice of Action, a receipt letter confirming that you have already applied for your work permit to be replaced.