Dear Anu
There may not be any standard format for the same, but assuming that reciept of your experience-cum-relieving letter has been delayed, you could use the following. "Dear Mr. <Sirname>, I had been employed with <Organization Name> in the <Department Name> department as <Your last designation> since <Date of Joining> till my resignation on <Your Resignation letter date>. I was relieved from my duties by my <Reportinga Manager's Name> on <Last working date> post my resignation & successful completion of full & final settlement procedures/handover of my responsibilities . However, till date I have not received my experience certificate or my relieving letter. May
I request that the same be made available to me at the earliest. Thank you & with regards, Your name"
Hope this helps!
Regards
Steney