Um... I have a silly question... how are you going to detect that the page is down when your computer is turned off? You would need a second machine. Ok, assuming you get past that problem, you just need to write a script that tries to get data from your web server every-so-often, and have that script e-mail you when it fails. This is not that difficult a task, but requires you to run a speudo-daemon or speudo-service to do the polling. Not exactly the best choice. (There may also be off-the-shelf solutions to do it.)

If you scan through the archives of the monastery you should find example scripts for sending e-mail from a script (using sendmail or SMTP) and hitting your server with a request is relatively straight forward. Good luck.