Hi @AnnaCazel,
What you are describing is something that randomly happens and I have not found a rhyme or reason to it. The problem can also occur with any compatible file in Canvas when using the DocViewer (which is used in the SpeedGrader but with additional functionality). Usually when it happens to me, it is while I am in the middle of documenting/demoing something and it is frustrating; so I understand how you feel. Whenever I have reported it to Canvas Support, the problem either resolved itself a few minutes later or they told me (after researching it) that it was a temporary glitch.
I know this will not ease how you feel, especially when grading more than 200 submissions, but I would recommend that you just stop for about 5 or 10 minutes and then return to it or switch to a different assignment. If the document/submission does not load after that, you can then report it to Canvas Support or to your Canvas team.
-Doug