Telegram - My phone got stolen (or lost), now what ?

My phone was stolen, what do I do?

First of all, sorry about your phone. Unfortunately, the phone number is the only way for us to identify a Telegram user at the moment. Telegram doesn‘t collect additional information about you, so whoever has the number, has the account. This means we can’t help you unless you have access either to the phone number or to Telegram itself on any of your devices.

If you have access to Telegram on another device:

1. Go to Telegram Settings — Privacy and Security and turn on Two-Step Verification. This way the phone number alone will not be enough to log in to your account.
2. Go to Settings — Privacy and Security — Active Sessions and terminate your Telegram session on the old device. Whoever has your phone will not be able to log in again, since they don't know your password.
3. Contact your phone provider, so that they block your old SIM and issue a new one with your number.
4. If you decide to switch to a new phone number, don't forget to go to Settings, tap on your phone number and change your Telegram number to the new one.

I don't have access to Telegram on any other devices:

1. First and foremost, you need to contact your phone provider, so that they block your old SIM and issue a new one with your number.
2. Wait till you receive your new SIM with the old number, log in to Telegram, then go to Settings — Privacy and Security — Active Sessions and terminate your Telegram session on the old device.

Removing sensitive data

Common thieves usually throw out the SIM card immediately (the phone is harder to locate this way), then wipe the devices and sell them, so there isn't much risk for the data in case of regular petty theft. But if you have reasons to worry about the data on the device and are unable to log out the other device, it is best that you wipe it remotely. You can read more about it here: Apple iOS, Android. Unfortunately, this requires you to have prepared in advance for this scenario.

You can delete your Telegram account if you are logged in on at least one of your other devices (mobile or desktop). Note that inactive Telegram accounts self-destruct automatically after a period of time — 6 months being the default setting.

Last but not least

My final advice is to always install Telegram at least on two devices (ie. your phone and your laptop/desktop/tablet) and activate the Two-Step Verification, just to be safe.

REF:  https://telegram.org/faq#deleting-contacts-on-android