The warranty will depend on where you are and when you bought it. Within the EU it's a 2-year warranty, most other places will be 1-year. If you're within warranty then you can return it to Nokia for replacement (they don't seem to offer repair for this issue). They've pretty much run out of N900s though, so you're likely to get an N8 (or E7 if you push) back instead.

Third-party repair will depend exactly how it's failed. If the port has just come loose then a repair should be fairly straightforward for someone with the right gear and level of expertise (I wouldn't recommend trying it yourself unless you're really skilled), but if it's torn off altogether then it'll probably be a lot more tricky (if not impossible).