Hello,
The most common source of issue is when the Gamebuino is accidently turned of while loading a game, resulting in a corrupted program in the memory. This can prevent the console from starting or making it act weird. The solution is to hold "C" down, turn the console on, release "C", and wait for 30 seconds. This will force the Gamebuino to flash the program LOADER.HEX from the SD card.
If this doesn't solve your problem, you can try to format the SD card and re-add all the games back. You can download all the .HEX at once from my
games compilation.