I would close the client and then reopen it to see if that resolves the issue. If not, then I would go into settings -> Dynatrace Server settings and test the connection to the Dynatrace server.
Are you the only user to experience this issue, or is it spread across your entire user base?
Edit - the reason I ask is because if it is wide spread it could mean there is an issue with your frontend server where it might have lost connection with the backend server. Restarting it might resolve this issue.