Wow. Sounds like you have quite the challenge ahead of you. You might need to have the exiting IT guy stay on for a few weeks to help with the transition, if that is not possible check for local IT consulting firms. One of them should be able to get you up to speed and help you with support until you can carry it on your own. Some research on backup systems might also be in order.
In desperation, in off hours, reboot the server using one of the fine “forensic” Linux distros. Now that Helix has gone commercial (http://www.e-fense.com/products.php), its not an appropriate suggestion for this specific task. SMART (http://www.asrdata2.com/) is a useful alternative. (I’ll need to try Penguin Sleuth http://www.linux-forensics.com/).
Your task is to gather information about the server and, if at all possible, have the departing IT person tell you about when you see.
Seriously consider replacing the server with one whose history you can vouch for. This is always important, and even more so if the departing IT person is uncooperative.