My facebook is saying "Facebook will be back soon" for three days now, on my mobile device and laptop. Someone help!
You did not leave very much information (ie. Type of device...Apple / MAC, Android, Windows) but here are some generic things to try.
Delete your app from your device. When you reload the app, make sure it is the authentic FaceBook app made my Facebook and not a 3rd party.
If you are running a program on a computer run a virus scan which includes FaceBook Virus protections. Clean out any saved cookies and passwords and try to log in again.