Hi,
There is several possible reasons for that to happen, I'm actually thinking of 4 possibilities :
1- Maybe Google did a rollback in the past due to Algo updates or the SOPA protest. In which case, it should be back within a few days to 1-2 weeks.
2- A Penalty on domain level. Did you see a huge drop in traffic in Analytics ?
3- Google decided it wasn't creating any value so he remove all the link at once, which would be weird all at once.
4- Change in the URL structures on the site which you probably would be aware.
Did you have those sitelinks for a long time ?
Sometimes these types of changes are from Google testing and implementing of new stuff. Google seems to be doing a lot this month with the Google Plus your World they put in place on January 10th, they did also played with Crawler Bots speed for the SOPA Blackout this week, I would on first tought without seeing anything, tell they probably Roll Back a bit, so this should fix itself alone.
By the way SOPA is evil !