@Heena can you log in to your self serve account and click on lost /stolen to deactivate it for now? If it's an iPhone or Android and it's set up the right way you might be able to find it through find my iPhone or find my mobile!
If your Phone is lost or stolen, you can Suspend your service if you’d like to prevent any calls or messages from being sent or received. Doing so will not keep your plan from expiring; however, it will ensure that your credit card is not charged at your next renewal date if you’re on AutoPay.Just follow these steps:
Note that after 90 days in suspend, your account will be deactivated permanently so be sure to return to this page if you find your phone or decide to get a new SIM card.
@Heena Have you tried phoning your lost phone to see if someone answers before you report it lost. I've found a phone before on the street and well in a restaurant close by the person called, I told them where I was and they were happy to get it back.
Unrelated but if you want to retrace your phone's last steps follow this guide to download your Google history (generally you are logged into Google on your phone)
Hope this helps