Help! Upload folder is not writable. Export and file upload features will not be

Hi,
I’ve just logged in to the back end of my site for the first time in a while and am getting error message: Upload folder is not writable. Export and file upload features will not be functional.

I have not made any changes to my host or site. Can anyone please tell me how to fix this? I am not an expert so grateful if any advice could be kept simple!

Many thanks.

http://www.boatfishingcharters.co.uk/