The first day back at work can be daunting, but you can get through it without going nuts.
If possible, start just a little early and take a quick pass through your incoming email, papers, voicemails.
Remember the Getting Things Done principles: you'll be less harried if you have a clear head and a good picture of what's on your list. First collect all the incoming stuff and process it quickly – less than 2 minutes on each thing – to delete or file that which requires no action on your part, note upcoming commitments on your calendar, delegate that which needs to go to someone else first, and otherwise identify things that need more of your time.
Prioritize those things – ideally by sorting/labeling as you've gone through them processing & organizing – and then just start plugging through them.
Sudden stuff will come up. If it really isn't higher priority than other things in your queue, then just prioritize it and do it at the appropriate time, giving whoever is bringing it to you a realistic prediction of when you'll get to it (and if needed an understanding of the stuff that has to get cleared from your plate beforehand).
Chug on through. Maybe you won't get very far down the list, but you'll know what's going on and be working on the right things first instead of just the noisiest or most fun.