Maybe nothing. First, be sure there is nothing wrong, like diapers, ear infections (which may not have other symptoms) or hunger. I assume they have not been doing this before? If it is new, then have their doctor check them over after you have eliminated the diaper/hunger issue. If they have never settled themselves down in the past, then you may have to teach them how to settle. It is hard, but you can do it!
Go down the list. It's good to have a system-- checking to see if they are hungry, wet, or poopy is usually a good place to start. If those things aren't working, you can repeat your bedtime routine (snuggle, read, etc.) and see if that helps. Check their temperature to see if they have a fever (you can give Motrin or tylenol). If they keep crying and can't be comforted, then you need to see your doctor.