Hi AnneC,
A warm welcome to our community 
I will be happy to help find out what's happening for you. Firstly, how are things looking today? If you're still experiencing this issue then let me know.
Also if you can let me know if the HDMI lead is plugged in securely? It may be worth removing the HDMI lead and putting it back in to ensure it is secure. If the issue is persistent, keep me posted.
I'm here to help if you need me.
Collette