1) next time, add an entry to lilo.conf instead of modify old one, then u can boot old kernel if the new one does not work
2) u need to use mkinitrd command to generate a initrd-2.4.20.img for u new kernel. by search initrd in this forum to get more information.
or u can comment that line 'initrd=...' and modify the root=LABEL=/ to root=/dev/hdxy. check u /etc/fstab and replace that hdxy with the partition u mark as /