I’m running out of ideas.
Do you have the screen wired up correctly?
Can you try the blink example (File->Examples->01.Basics->blink)? It should blink the onboard LED on D13. I would disconnect the RAMPS board before running the blink. I don’t know if it has anything connected to D13 or not.
The simple answer is that it replaces the previous code, it doesn’t add it. The arduino only runs one program (that is an oversimplification that is ignoring the bootloader, but you are replacing whatever software was there before.
You might also try to connect the USB to your computer, and connect using repetier host (250000 speed) and see if that works, which would tell you the program is installed, and maybe the screen just isn’t working.