Losing access looks the same from the outside — the account will not let you in — but the causes differ and so do the fixes. In order of how often they happen.
The session was terminated
The most common case when running several accounts. A session stops being valid if it was ended from another device, if the account was logged in again elsewhere, or if Telegram considered the login suspicious.
The fix is to log in again with a code. The code arrives inside Telegram on another active device, and only falls back to SMS if no active session exists.
The number is gone
No number, no code, no account. This is the main reason accounts are lost permanently and the main argument against cheap disposable numbers: they are resold, and the account goes with them.
The only real defence is keeping at least one active session on a device you control, so the code lands there instead.
The cloud password is forgotten
Two-step verification without a recovery email means a lost password can only be resolved by resetting the account — a week of waiting and the loss of message history. With an email attached, the password is recoverable. So: turn on two-step verification and attach the email at the same time, not later.
The account is restricted, not lost
A separate situation: login works, but messages to strangers do not arrive. Nothing needs recovering — the restriction needs lifting. See why Telegram restricts accounts.
When nothing can be done
A deleted account does not come back, whether you deleted it or the platform did. The number can be registered again, but that is a new account with no history, and it starts from warm-up: warming up accounts.
Prepare in advance
Keep session files safe, keep control of the number, enable two-step verification with an email, and do not load one account so heavily that losing it stops the work — how many accounts you need.
How Neurogram does this
Everything above is manual work that runs into the number of hours in a day. Neurogram does it on your own accounts and without you: it collects channels and audiences, writes comments and replies with a language model, and keeps limits and quiet hours so the accounts survive. See pricing or look at your case.