[help]While I use qemu and gdb to debug a kernel, it stucks in "[0]kdb>" and can't enter anything

gdb show that the statement that is excuted is
93			ret = nb->notifier_call(nb, val, v);
and qemu just says "[0]kdb>" but i can't type a word in it.

When I run :

qemu-system-x86_64  -kernel /home/mm/kdb-work/bzImage  -append
"root=/dev/sda" -boot c -hda /home/mm/kdb-work/busybox.img -k en-us

The qemu starts but it seems that there is a problem because it says
"Kernel panic -not syncing:No init found"

I want to know if this make the problem?
Does any body can help me? Thanks a lot