Have you seen a popup on your iPhone or iPad claiming that iOS has crashed and that you need to phone a helpline for a fix? It's a scam, do NOT call the provided phone number.
Here's the Fix:
Put your iPhone or iPad into Airplane Mode (go into Settings and toggle the Airplane Mode switch into the off position).
Go to Settings > Safari and tap Clear History and Website Data.
Close Safari (double-press the Home button and swipe Safari up to close).
Exit Airplane Mode.
To reduce the risk of this sort of issue from happening in the future you can block popups: go to Settings > Safari and toggle the Block Pop-ups switch.