I carelessly lent my N800 to someone, and he yanked on it against the power cord.

Now the power plug is broken.

I'm sure it's not the power cord, because my other power adapter is not broken.

How do I fix this?

If its the plug in the Nokia its self sounds like you need to contact Tech Support and send it off for repair.

If you are out of warranty you might think of:

If you are VERY handy you might try and repair it yourself... I repaired some jacks on phones and it's not that complicated once you know what you are doing... You can always google on how to repair a power jack on a phone. (try new nokia's they have the same plug as the nit). But chances are that if you didn't think of this solution you don't have the tools needed to do the repair.

Another solution would be to buy a desk charger something like this one. I am sure I also saw one that can charge two batteries and no phone.

Good luck !

Ah, no, I thought of repairing it myself, but decided against it because I lack the tools and the parts, and figured the people at Nokia would do a better job.

I sent it back to Nokia, and they repaired it for no charge, because it still had a couple weeks of warranty. The repair ticket says the technician just adjusted the power port, which still must have involved taking it apart.