SOLVED! - Suddenly can't upload files for download from link

SOLVED: OK I worked it out!!!

I was mistaken when I thought the failed file was smaller than the others. In fact it (and of course the big pack) are just over the 20MB limit for uploads. I wish it gave an error message and explained why it was failing. 

My original message:

I'm making a set of .zip files available for download through text links and buttons. 

I did a load of them no problem, but I realised the first hadn't worked. Tried again but it just hangs when I try to upload it (have attached image - it just stops here, with the line moving under the filename as though something is happening, but it never completes).

It's smaller than the other files, doesn't have a long or weird file name (I even tried making it shorter), no weird characters. I deleted a load of unused files, cleared my cache, restarted my computer, it just hangs. I tried a different file and that hangs too. 

Also side-issue: I can't see the files in the assets list - only images - where are they?

Please help! I need to launch this website asap!

Update: I just managed to upload a different, smaller file. 

The two that are failing are a  sub-pack* .zip of .pdf/.png files, and a full-pack .zip that includes all the files I've successfully uploaded as sub-pack .zips, plus the .zip file that isn't working. 

I tried to make the full pack without the smaller sub-pack that failed, and that failed too. So I don't think it's anything weird in that file. 

My best guess is that I've run out of file space?? Although  I haven't got a load of stuff on there so this would be a surprise.

Questions

- how can I confirm if that's true?
- how can I see how much space I have left? 
- how can I see previously uploaded files to delete old ones?

* I just mean it's like a smaller pack that is included in the full pack, this isn't meant to be tech terminology or anything