If you're having trouble uploading your explanation, check to see if the problem has to do with uploading an attachment. If it does, submit the written lesson without the attachment, and then send the attachment in a message to the student (be sure to do both at the same time, you can't send the attachment via message later).
If you aren't trying to upload a file, try refreshing your page or switching to a different browser. If you're still running into issues, it is likely that this written lesson was passed off to another tutor. This may be a result of you taking too much time on the explanation, or you leaving the explanation page by viewing other tabs on your browser.
You'll know if the written lesson still belongs to you, because it will clearly indicate that on right side of the written lesson:
If you navigate away from the written lesson page, or take too long providing your explanation, the written lesson will be sent out to other tutors. This means that the lesson no longer belongs to you, and there is no way for you to answer it. The written lesson will have no option to submit: