Hello aswin pp,
Certain SD cards have trouble being recognized by the AM335x SoC's romcode, depending on the formatting of the SD card. If the procedure in the user guide to format the SD card did not work, please try another SD card of a different brand and size.
You can also try to set the bootable flag manually. Use a raw sector editing program such as sectedit (http://www.roadkil.net/program.php/P24/Sector%20Editor) to change the value of the byte at address 0x1BE in the first sector of the SD card. Here are some rough instructions to follow:
1. Install sectedit.
2. Insert your SD card.
3. Launch sectedit, select the physical disk corresponding to the SD card.
4. On the first sector, find row 0x1B0 and column 0xE and verify that the byte's contents are 0x80. If not, change it to 0x80 and save.
5. Try to boot from the card again.
Regards,