At the very least, I'd expect large-scale protests/civil disobedience if Trump refuses to concede. I think it'd mostly take the form of pressure on Republican state and county governments to declare a Clinton presidency illegitimate. You could expect it to look roughly analogous to the Nullification Crisis in 1833. We could see the heavily Republican states start to openly defy a Clinton administration. Where it goes from there is anyone's guess.
HSLD