Problems uploading photos to media library

I was in middle of composing a post and started adding photos. 3 worked and then I suddenly kept getting the message, “[filename] has failed to upload due to an error.” The max file size is 8MB, and they were far, far below (one was only 565K).

I’m not sure why it stopped working in middle. Was I doing something wrong? Can I fix this?

Thanks in advance!