Press small middle button on "top" side of N900 and keep it 4 secs. It is a fast way. Regular PC works in the same design.

If phone works - a short press you a menu with position "switch off!". Of course, unlock first or it will ask you to swipe unlock before a menu.

Tryng to connect to WiFi again - after phone loads it would connect automatically (depending from your setup) or you can press small battery icon on top of screen, then select from menu "Internet connections" and select your WiFi.