Hi, this is more of a Raspberry Pi question than a RetroPie question, and would probably be better off on a forum related to your question in the future. However, since it’s up here, I’ll see if I can help. Are you talking about booting Rasbian for the first time? If so, make sure you format your SD using the SD Formatter on SD’s website. Make sure you run a complete overwrite. Then, if you’re trying to boot from noobs, make sure you unzip the file and move all of the contents of the file to your SD card. There should be nothing but these files in the SD card. I hope I helped your problem. If I didn’t, please let me know and I’ll help if I can.
Reply To: RPi not booting after writing image to SD card