Dear Anu,
There may not be any standard format for the same, but assuming that receipt of your experience-cum-relieving letter has been delayed, you could use the following:
"Dear Mr. <Surname>,
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 <Reporting Manager's Name> on <Last working date> post my resignation and successful completion of full and 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 and with regards,
Your name"
Hope this helps!
Regards,
Steney