loader prompt doesn't work

Dear all:

     A quick question for loader prompt in FreeBSD OS.

     Recently i do some benchmarking in the FreeBSD OS, and today i decide
to enable some

     performance tools in the kernel which can be realized by building a new
kernel.

     Before that, i refer to the FreeBSD handbook for howto.

     It says that if i fail to load the new kernel, we can select the
"escape to a loader prompt" option.

     By loading the correct kernel,  we can reboot the system.

     However, problem happens. When i select the "escape to a loader prompt"
option, i can input

     in about just 3 sec. After that, the system cannot go on whatever key i
hit.

     The whole system have 4 hard disks; and i install 3 OSes in the first 3
hard disks which are

     Linux, Solaris and FreeBSD in order. All systems are booted using grub.


     Any ideas?



                                           Best Wishes!

                                                     Yan