They can do a number of things: garnish your wages, take any income tax return you might otherwise receive, and at the worst, you could be put in jail.
I have just gotten my student loans under $40,000, but I had a lot of years where I couldn't make the payments and I had to do forbearances, deferments, etc. to try and keep from defaulting on my loans. I met with a credit counselor who told me that the student loan debt is more important to try and pay back than credit card debt because the credit card companies can't do much of anything to me if I didn't repay them, but there are some strong penalties for not repaying a government loan.