First, ensure you have an internet connection.
Then try the following:
On mobile app: Sign out of the mobile app and sign back in. This may help to clear the "cache" on the app.
If this does not resolve your issue, you can try any or all of the following:
Disable ad blockers or other browser extensions or add-ons.
If none of these techniques work, we recommend submitting a ticket to edX:
On desktop browser: Click the Help link on the top of the edX screen.
On mobile app: On the Courses page of the app, tap the "menu" icon, and then tap Submit Feedback.