A PDF, video or resource won’t open

This problem is almost always on the browser-side. Work through these in order.

Clear your browser cache and cookies, then sign in again. Try a different browser. Disable extensions, especially ad blockers, script blockers and PDF viewers. Try a different device.

If it's still not loading, tell us the exact course and resource, the error or what you see instead, your browser and device, and whether other resources in the same course work. If it's one resource failing and the rest are fine, that's a different problem from nothing loading, and knowing which saves a round trip.