A random game loads when phone is unlocked?

I have a UK note 9 and recently, when I open my phone from being unlocked a random game starts to play and does not go or finish until a x mark turns up in one of the corner of the phone screen to cancel it, which can take up to a minute of time, the phone is useless up until then, how do I get rid of it. Please send details, thank you

Welcome to Android Central! Assuming this game isn't actually installed, it sounds like a popup ad. See this guide to figure out which app is responsible: Full screen ads. How to find the culprit. - Android Forums at AndroidCentral.com