Thanks for posting here in the Google Nest Community.
I know how it feels when something isn’t working as it should. A few things: which Nest display are you trying to cast the video to? Which video service are you using? Were there any recent changes made before the issue started? What steps have you taken to fix the issue?
You can try a sequential reboot. First, unplug your Nest display for 30 seconds, then do the same thing to your Wi-Fi router to refresh both devices and your Wi-Fi signal.
Let me know how it goes.