I thought Nemesia was an annual plant. So you will need to resow some seed or buy new bedding plants in April/May.

Guest, there are perennial hybrids of nemesia, but their frost tolerance is strictly limited. Try searching the plant on the web to see which climate zones it's recommended for. If your climate fits, prune off all the dead stems, right down to the ground, and give a light application of fish emulsion and seaweed extract. You might want to wait until your average frost date to do that, though. I get the impression that the UK had exceptionally heavy frosts last winter, so even though it's supposed to be hardy in your neighborhood, it may still be kaput (that's a gardening term.) : D

Good luck!

I have lost my perenial Nemesia as it got just too cold for too long.