Node library has disappeared. What to do?
Please try updating your Revit version as this is most likely a TLS issue - Have a look here:
Hi my friend! Dynamo Core Sandbox not working with Revit
What version of Sandbox are you in? Likely the above TLS link will still fix it (If not one of the later versions of Sandbox).
Check the logs too.
If the node library is gone, Cef is not loaded, see the wiki page:
TLS would effect package search, not the node library.