Not much to go on...could use some more details. How long ago was the escape, when did you first notice?
They don't get far most of the time, start by searching around the immediate vicinity of the enclosure. Any dark spaces will attract them, check there first. Shut the door to other nearby rooms too, block the bottom with a towel or something else so it cant get through.
Search everywhere, and leave no stones unturned until you find it.