Rob,
Do you know what the IT company have done to it? And more importantly, is a recent backup of the database available?
It may be fixable, or if the DB is fine, possible to reinstall the Drupal software and whatever software plugins were added and then import the database to put you back (hopefully) to the pre screwed up state.
Installing and getting started with a PHPBB setup in its basic form is trivially easy - as long as you have the information on where the database lives then you can be up and running in a matter of minutes. Hell, I set up a forum on one of my domains once to discuss something with mates, simply because it was a quick way to deal with our wildly disparate working hours. There are even (to my knowledge at least) a couple of scripts for importing Drupal data into PHPBB, so you can keep users, posts, etc etc and go from there.
Where it gets a bit more complex though is tinkering with things to add functionality or change the look of the board. There's a huge array of extras available, thankfully most of them play nice together and are well documented enough that you don't need to be a genius coder to get them working.
I have a feeling though, I've asked Jay before about our calendar only to be told it's something of a bodge job - it works, and well, but definitely not a simple thing to replicate.