Ubuntu won't suspend anymore, but it did upon install

Open your terminal and type this

sudo apt-get install hal

Open the Software center and type 'acpi-support' in the search bar. Make sure, that the package is installed. Reboot, and voilà: suspend and hibernate are back and working again for me. Maybe this will fix it for you, too.

I know this is a bit late, but I am having the same problem as you.

It turns out laptop-mode-tools automatically removes pm-utils upon installation. I'm not sure why, but it would seem the 2 do not like coexisting.

Try installing pm-utils again with: sudo apt-get install pm-utils

For reference, I found this out from: http://ubuntuforums.org/showthread.php?t=1660478

I hope this helps to solve your problem.