Grey screen when waking from sleep mode

[MaxStorm82]

Hi, 

Between long breaks of not using my phone after it goes into sleep mode, when I press the Power Button it shows a grey screen for a few seconds and then goes back to sleep.

To use the phone, I have to restart it by pressing and holding the Power button to get the Power Off and Restart menu.

This has been going on for the past 2 months.

Also, I sometimes get an error message showing "Unfortunately, the System UI has stopped working." 

What should I do about these problems? There is no water damage, but the phone did fall on the floor a few times in the last two months. 

Please let me know how to fix these problems. 

MaxStorm82