The stock market runs in cycles, and it's madness to acknowledge this and then seek to cash in on your loss in a market downturn.
The downturn is simply that, but unless you desperately need your money now, there's no pointing cashing in your loss.
Just hold on as the inevitable upturn comes about over the next few years, and watch it grow back.
I can say that as someone who's seen his ISA wiped out by 80% - but I'm not worried - I'm in this for the long term.