fix a problem with image uploads?

​Hi,
I am currently having an issue with images on my Drupal hosted website that I could do with some urgent help on if possible.

I was originally having the issue of not being able to upload any images with an error message of 'The file could not be uploaded.' appearing.

From what I found on the help forum, it said to go into configuration/media/file-system​ and that there may be an issue with the 'Public file system path' which was set to 'sites/default/files'
This had the error message of 'The directory sites/default/files exists but is not writable and could not be made writable.'
I tried to change this but now no images on the website will appear at all on my company website!!

Anyone know what I need to do to get this back up and running so that images will show again?

(I'm not exactly that tech savvy so a dumbed down version would be welcomed!!)

Thanks!