Not much you can about it now - it'll probably regrow, so I wouldn't worry too much. In future wait till its actually spring (March or April) and you can see new leaf growth at the base, cut it back then. Despite some nice weather recently, the temperatures are way too cold for cutting back this, and other, plants.

With tall ones like that if they look a mess and annoy you, another year you could compromise by cutting them half way down -then there would be enough left to protect the crown if necessary. The penalty is having to do the job twice...

They are tough as old boots - not much chance of them being dead. It's too early for any signs of growth yet.

Thank you for all your advise keeping my fingers crossed as a few plants eek - thought I had been advised to dead head and cut back but just hoping they will regrow!!