Bootloader failed (BeagleBoard Rev C3)

I have a BeagleBoard, Rev C3, it can not be booted after I run a commad:

nand erase
reset

I try to use MMC card to boot by press ‘USER’ button, it does not work.
There is no message output via the RS-232 console.
How can I rescue the board to boot successfully?

Need your help!
Thanks!