Admin log in no longer working

[toppop12]

I'm running 1.1.20

Two weeks ago I had no problem logging in with my admin user and password and now it no longer works. I try to log in and it just loops back to the log in page and says 'You Should Fill in a Username'. I tried to do a lost password/password reminder, enter my username and I get this message:

An Error Has Occurred!
Your session timed out while posting. Please go back and try again.

I have a second admin and just tried his user/pass and it does the same thing. Cannot log on. Also, the Admin tab is no longer showing up either. 

Not sure what happened or what to do?

[Jade Elizabeth]

What was the last thing you changed?

Perhaps run a repair-settings to see if you can get in with the default theme?
http://download.simplemachines.org/?tools
What is repair_settings.php?

[margarett]

It would suggest something changed at server level, though. Or an incorrect session duration

Can you get us a phpinfo?
What is a phpinfo() file?