Occasionally things break or get blocked.
A quick fix can be to refresh the page. Try clicking on the document again and hopefully it the preview will load and you're able to download it if you wish.
Still not working?
If nothing is loading and you are seeing screens like the ones below, then it is likely that it is being blocked by your company firewall.
We can help talk to your IT department to fix the problem, or you can point them to this link.