The green ACT LED (beside the audio socket) is the equivalent of the hard disk activity LED which used to be on the front of all PCs. If during the first 20 seconds it flashes briefly just once or not at all then the RPi is not booting. Period.
Possible reasons for above:
1. Bad power. (PSU or cable)
2. Wrongly written or damaged file system on SD card.
3. Firmware on SD card is too old to support the RAM chip on the RPi.
4. Card socket fingers not making contact with all pads of the SD card. Physical damage, bent fingers, curved SD card etc.
If you cannot get it to boot anything I suggest you read the stickies in the troubleshooting section of the forum at
raspberrypi.org.