Try clearing your browser's cache and refreshing the page. Most of the time that this happens, it's a network issue and resolves itself in a little bit. Clearing the cache can help remove any stale portions of old code and make sure it's getting the latest version including any updates.
Hi email@example.com, James made an excellent point.
Here's some information about how to clear your cache:
Sing out if you need a hand.
We have seen this happening recently with some students that were using Kaspersky antivirus.
For some weird reason it was blocking some HTTPS URLs, including tha Canvas core CSS files.