Why my Facebook cannot log in?

Any error messages.
The more information you give the better we can assist.

I agree with Ben, but also have to mention that FB (facebook) has been having a lot of problems lately.

They're constantly adding things to the site which can often cause problems.

Usually if you wait a few hours, the problem is fixed. Until next time. :)

First, make sure you are going to the official Facebook address (http://www.facebook.com).

There a lot of sites out there designed to look like Facebook, but are just trying to steal your login information. Always login from the main page, and never anywhere else.