Pls help me to restore my n900 stock kernel when i try this command

apt-get install --reinstall kernel

error message pop out it says:

"E: could not open lock file /var/lib/dpkg/lock...
E:unable to lock the administration..."

I want to restore my n900 kernel from flashing in to linux kernel 50 because it caused my phone some defects i do not usually encountered like unwanted reboots..

Pls can any one help me or some advise??