Happened to me. No obvious reason why the condom failed that time.
Took Plan B right away. This was back in the day before Plan B and other emergency birth control was available w/o prescription. Now you can get Plan B without prescription. Most pharmacies sell it. You might have to look for it on the shelf and/or ask the pharmacist directly about it. Even though you don't need an Rx, most pharmacies keep it behind the pharmacy counter because it can't be sold to those under 17.
BTW, see the recent NYT article on Plan B, which says that it acts to prevent pregnancy by preventing ovulation and not by interrupting implantation of a fertilized egg. The latter would be troublesome to pro-lifers, but the former shouldn't be. See
http://www.nytimes.com/2012/06/06/health/research/morning-after-pills-dont-block-implantation-science-suggests.html?ref=planbcontraceptive
Of course, Plan B is more effective the sooner it is taken after failed contraception. I think there is a 72 hour recommended window, but even w/in that, the sooner the better. I keep a box in the closet now. Thankfully, I've never had to use it again.