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Try to update first and foremost.

In order to not lose your secret chats you should under no circumstances uninstall if there is a chance that an update might fix it. If you have to use Telegram in the meantime use another client like Nicegram which is based on the regular Telegram iOS app.

You should also submit a card on https://bugs.telegram.org/

You did what exactly? If the issue is not fixed or acknowledged in the changelog of recent updates you should submit a card in the link from my first reply to let Telegram know.

If you Telegram application still doesn't work there isn't much you can do, sometimes these things just happen and it's unfortunate.