Once Kernel is built wrong, it can't boot up, or you delete /bin/ some files by accident, nothing can run, even ls command, DON'T BE panic.. You can use tftp to reboot from other PC.
First you have one Linux PC which tftp is installed and enabled.
Then you can put compiled Kernel and root file system to Linux PC, BBB board can boot from this PC with Kernel and root file system.
After BBB board is powered on, press enter key quickly, it will enter u-boot mode, you need to set BBB board local IP address, Linux PC server IP address and tftp boot command. Something like:
Linux PC IP address:
setenv serverip 192.168.2.99
BBB board IP address
setenv ipaddr 192.168.2.97
Linux Kernel, you need to put Kernel uImage file at /tftpboot
tftp 0x80200000 uImage-dtb.am335x-boneblack
u-boot will download Kernel image to BBB board.
Set root file system, nfs service is installed and enabled at Linux PC
setenv bootargs console=ttyO0,115200n8 root=/dev/nfs rw nfsroot=192.168.2.99:/home/bsang/sang/robot/bbb/rootfs ip=192.168.2.97:::::eth0
boot BBB board
Congratulation! Your BBB board is still working. You can new Kernel to BBB or you can upgrade root file system. Your life is saved.