Facebook may be down – Occassionally the servers will go down and at that point you will not be able to visit the site at all. This is not a problem on your end, and simply an issue with the computers that you are trying to connect to. The best thing you can do is try to refresh the Facebook homepage (press the F5 key) and wait until it comes back up. You will know if this is the issue if your web browser gives you a “could not connect” error.
You have a malware infection – Sometimes you are surfing the internet and you pick up various spyware and malware along the way. Spyware and malware are bad programs that are designed to mess with your internet and make you view pages that you do not want to visit. If you are seeing an abnormal amount of ads or if you frequently try to visit a page and get tons of popups, you may have a spyware or malware infection. Get a friend who is experienced with computers to clean your PC.
You are using the wrong info – Make sure you are using the right username and password. Facebook will only allow you to use an actual email address for the username, so it wouldn’t simply be something like “myusername”. It would have to be something like “firstname.lastname@example.org”. If you have forgotten your info but still know your email address, you can recover your data by visiting the Facebook help page.
Your account has been hacked – If you are pretty sure you know your login details but they are still not working, there is a chance your account have been hacked, cracked, or simply jacked. What happens here is usually that you have set a weak password and somebody else is able to guess the information. You can always find out if your account has been hacked by calling a friend who is your friend on Facebook. You can ask them to check and see if your profile has been doing anything weird, such as posting spammy links or sending out weird messages to people. This is often a sign of a compromised account. If this is the case, use the help center at facebook.com/help and used the Hacked Account help page to unhackerize your account.