Things to check:
SD Card: Has it been tried before or verified?

Keyboard: Try plugging in another keyboard to see if something may be stuck

Power Supply: RasPi can be touchy with power supplies.

Graphics Memory Split: What are you set at?

Command line attempts: Go into the EmulationStation systems config and try to manually input the commands to start a ROM through it’s specific emulator.

EG: The command line syntax I am attempting from the Osmose directory is:
./osmose /home/pi/RetroPie/roms/mastersystem/A.sms -fs (where A.sms is the ROM name… it may help to rename the ROM to something w/o spaces or special characters)