A small script that repetitively pings a server in order to find out when it goes back online.
Just execute the script version you want with two parameters: the website you want to ping and the delay time in seconds between a ping and the other.
Open the email version of the script in a text editor and set up an email address and an email server so that when the server is online the program can alert you properly.
You need the notify-send package installed for the notify version.