Linux – What are some useful utils when using Gentoo Linux

If you are using GRUB as a boot manager, stop the boot at the boot menu. Highlight the kernel you boot into and hit the 'e' key to edit the line. Add the option "single" at the end of the line and it will boot into single user mode.

If that fails, boot into a LiveCD and use that environment to mount your drive and fix the necessary files you need in order to fix PAM

For ongoing monitoring you could consider installing munin. It will gather information every 5 minutes and generates graphs that will allow you to see where the bottlenecks are. I also use sar which can be run in background mode gathering data to disk. This gives quite detailed infomation on what the bottleneck is. To what processes where running in the past you will need the process accounting package.