It could be they are still in hibernation ore if it is a small colony they only come out at night. Don't worry, if they didn't escaped they are probably doing fine.
I saw around 5 lasius niger ants today and yesterday going around in my garden ;) I have taken some of my colonies out of hibernation as they had eggs before they started hibernating and there still there did they survive or are dead?
It could be they survived, if the temperature stayed above +-2°c the eggs don't die. If they are dead the workers will take them out if they are awake. Don't worry the queen will lay news eggs soon. ;)