Why does my screen flash and go black when I try to wake it up?

When I try to wake my galaxy s5 using the home button or the wake button, the screen will flash white, then show lock screen very briefly, then go black, but the screen is on(responds to touch) because my soft buttons on either side of the home button are illuminated and I can swipe the screen and it will unlock even though display is black.

I usually have to keep toggling sleep/wake button until if finally comes on. The problem seems to be getting worse as it is taking more and more toggling to get the phone to display normally. I fear it is just a matter of time before it never wakes up again...

I should also mention that the phone is over 2 years old and although I always keep it in a case, it has been dropped many, many times.

Any input as to the cause/solution to this problem would be greatly appreciated.

TIA