How to fix problem after changing IP address in Ubuntu

First, we need to install aptitude in case we don't have it
sudo apt-get install aptitude

Then,
sudo aptitude show ssh
to see if the openssh-server is installed (on our new PCs it wasn't after installing Ubuntu)

Install openssh-server:
sudo aptitude install openssh-server

ifconfig and check if the IP address in the eth0 connection is the same as in the IPv4 setting in the gui (it wasn't in the case of the new PCs)

Enable networking by right-clicking on the network icon at the top of the desktop.

Restart network manager
sudo service network-manager restart

Now, the IP from ifconfig and from the IPv4 settings should be the same and ssh should work.