Fastest fix first.
Force close the app and reopen it. On iPhone swipe up and flick the app away. On Android tap the recent apps button and swipe it off.
Still broken? Delete the app and reinstall it. You lose nothing, your account holds everything.
Check the app is updated in the App Store or Play Store.
Try another wifi or mobile data.
That solves most of these. If it keeps happening, open the chat and send your phone model and a short screen recording. We'll escalate it, this one's on us, not you.