How do I fix my sump nut problem? (1.6L Fiat Bravo 100 16V Troffeo)?

I went to change the oil on my Fiat 1.6L Bravo 100 16V Troffeo, and to my horror, as I started loosening the sump nut, I found out that it doesn't matter which way you turn, it's stuck there, not going in any further nor coming out; it's not in straight though, it looks like a thread was missed while screwed back in the last time the oil was changed. How can I take this sump nut out? Do I have to take the whole sump out and do it from the inside? I there a way to take it out from the underside of the car?
I've only just bought the car and I've never had this sort of problem.
Thanks for your help in advance.
T4Terry.
troffeo100.hatch@fsmail.net