It was working fine yesterday but now it just shows a black screen and crashes what do i do?

Actually the game isn't well optimised for mobile devices.....when I opened the game today and started playing , it was lagging in medium graphics & 30 fps settings !.....but when I played the game second time in Advance Graphics & 60 fps(And All settings were maxed ) the game was running smoothly ! ....So basically it isn't well optimised