Hard to say, depends when you planted them and how bad they are. If you cut them back or take all the leaves off now, they will die because they're unlikely to put out new leaves if the weather's cold. It will be a fungal infection - as to which, the link below might help you decide, and also decide whether to give up on them or not. If its not Pansy Downy Mildew, one or other of the fungal sprays mentioned might cure them and prevent new growth from succumbing. Don't plant pansies in the same soil again for a couple of years.

https://www.rhs.org.uk/advice/profile?PID=571