How do I set the computer to suspend when battery is critically low?

To change this setting back to suspend, use dconf-editor. Go to org.gnome.settings-daemon.plugins.power and change critical-battery-action to suspend.

Alternatively, in a terminal session use the command:

gsettings set org.gnome.settings-daemon.plugins.power critical-battery-action 'suspend'

The gsettings command is resident by default while dconf-editor requires installing via:

sudo apt-get install dconf-tools

I don't have sufficient rep to edit or comment, so if anyone would care to, this is a comment/edit for AtomHeartFather's answer.

You may also change the values for what percent is considered low/critical battery using dconf-tools. In the same power section, modify the following values to the desired percentages.

percentage-action

percentage-critical

percentage-low

Note: Special consideration goes to the percentage-action setting which will trigger the critical-battery-action setting.

There is no option to suspend available.

You cannot suspend when power is critically low, to suspend you need power to save the session to RAM allowing you to resume quicker, if you loose power you loose the current suspend state.

Hibernate copys the current state to the hard drive, it is slower but you can turn off the power.

Shutdown is obvious - you will loose any data in the active session and have a new session on restart.