Fix Wordpress Media Library Issue

General information for the website: Cannot add images to my wordpress media library or posts any more
Description of every page/module: I have a wordpress site which I update through the usual dashboard system. Since I upgraded to the latest wordpress version I cannot add any images to the media library or add them to posts etc. All I get when I try and upload them is a little box with a cross in it.

This is probably a really easy fix but I am not technical at all and only use wordpress to upload articles. I have had a couple of bad experiences on here with people bidding for work and then not getting in contact/not completing so please only bid if you can do the work and can do it straight away as I am looking to get this fixed very quickly.

Thanks in advance.

Vikki
Description of requirements/features: Url is www.thebookthefilmthetshirt.co.uk
theme used is Simplemag from Themeforest
Extra notes: