Can't upload anything

I was uploading thing fine yesterday, I don’t know what changed. I went to upload some images and the upload would start, and then stop. It stops at a different place each time, anywhere from 6% to 100%. Then I get the “IO error” message. If I try to use the browser uploader instead, I get sent to “[my website]/wp-admin/media-upload.php?inline=&upload-page-form=” which is just an error page (it depends on what browser I’m using). If I refresh that page, I get: “Are you sure you want to do this?” and that’s it. It’s not just media, I tried to upload a new version of my theme, and I had the same problem (of course the url is different, the error occurs at “[my website]/wp-admin/update.php?action=upload-theme”).

So far, I’ve tried using another computer: same problem. I’ve also wiped out everything except the wp-content folder and the wp-config.php file and re-uploaded EVERYTHING from a newly downloaded version: same problem.

I don’t really know what I’m doing wrong.

Thanks!