This might happen because of the Windows Focus Assist feature (called Quiet Hours in earlier Windows versions). When it's enabled, Telegram won't show new notifications because it adheres to that setting.
To disable the feature:
- Select the action center icon on the taskbar
- Select Focus Assist to cycle through the available settings: either Priority Only, Alarms Only, or Off. (If you don't see the Focus Assist tile, you may need to select Expand first.)