Experience certificate

what were your terms of service with them ,just check your offer letter/contract/appointment letter.

Since you failed to serve them with notice they may charge you someting. If you feel aggrieved and have not agreed to any term like prior notice before resigning  etc etc, then get an injuction from court and give them at their right place..

when did you took maternity leave. Also go through the offer letter/ appointment letter.
if it is mentioned that you have to serve notice period for specific period before leaving tje company, simply request them that you will join back and then serve the notice period.