well.. is everything else on your system working, is it just the Moodle? For a message like that, it usually means there is something else going on as well.

We are assuming that there has been no devestating attack from outside, or there has been no unusual event over the last 24 hours that would cause an issue. Is the Moodle on a separate server? Is that machine up and running? That is where you need to start from.

Go through the whole gamut of an entire system failure first, then if everything else is working, which is easy enough to check, then we can look at what to do next. 

I am assuming here that you have complete access to the server, the PHP, the database, the Moodle code, if you do not then you need to get that access, or work with someone who does.

First, check the server logs. Has some unusual event happened there? Your IT guys should be able to interpret the logs properly and let you know of any issue. 

Second, has the PHP logs been switched on? If they have, has something happened there that should not have happened?

Third, is the database intact? Can you access it? Can you see if the table structure is still OK?

I do not know your circumstances, but my guess would be that you have fallen into this Moodle admin by some strange series of events and until now, it has all worked fine. All these things above, sound complex, but they are the simple things, easily done, but important. How do you know if you patient is breathing? Major test simply accomplished - that category of thing.

Next, go into your sever and look to see if in the Moodle folder there is a file called config.php. If that is not present then you have serious security issues. Open the file in a text editor, NOT Word or Notepad, but a text editor, an open source text editor, llike Notepad++ and check it. Make sure that it has nothing between the line

<?php

Unset $CFG;

If there is, then you have major server security issues.

If all that is there, then try with another browser.

Sorry I cannot be of more help, I am flying out today, and be gone for a couple of weeks. No computers, no nothings... just R'n'R. Yeah, right....

Try the Moodle Docs, Beginning Moodle 2.x Administration is a good place to start, then the other Admin docs. 

Good luck and I hope this helps you.

It appears that your Moodle is remotely hosted on RackSpace servers.
IF that is correct, then IF all had been working a day ago, suggest contacting RackSpace technical support.
If I've been correct in sleuthing … did access the login page once … which showed site was in the maintenance mode.  So guess someone is in there and 'working on it'.  And, judging the theme (wood) am guessing your version of Moodle is 1.9.x or less.  Considering your moodle person left last year site *might* have been victim to a hack … or something as simple has hardware failure on the shared host system … or Rackspace might have had to do a move.

'spirit of sharing', Ken