You can try cutting down, but not before end of April, but you might want to try cutting down some of the stem rather than all - if it regrows after you cut it to the ground, you will have lots of tufty growth from the base and not a single stem. You might find, though, that the plants are actually dead, so if there's no growth anywhere by early May, they're goners.

thanks bamboo

I would give it like BB says 'till the end of April to decide if and how to prune it. Also yellow leaves can be a sign of lack of nitrogen so perhaps give it a feed. Is the growing tip still OK ?