- I was looking at a very important page, and then the server disconnected and the page is gone
- I went offline and shut my internet browser
Is the page stored somewhere locally so I can go and load it?
If the problem is your connection, or it's the server's connection and you get an error page from your browser (only if the server didn't propose a different page, which then the browser updates the cache, which you go to other solution bellow), then from the File menu (Press Alt to reveal the menu) click 'Work Offline' (also available from the Tools menu in the command bar), then follow the instructions on Al Everett's answer to find the desired page.
If you have a connection to the Internet but somehow the server is off and you don't have the page in the cache then there is another useful way I just found - loading the page from Google Search results' cached pages:
Ctrl+H is the default key combo to pull up your browser history. Unless the page you were viewing prevented caching and/or was dynamic, you should be able to get back to it.