After reading other posts on the forum I've tried:
1. plugging the monitor into a different computer. Same problem, so monitor problem.
2. plugging a new monitor into the computer. New monitor displays home screen fine, so monitor problem.
3. a hard reset on the monitor.
4. a hard reset on the monitor and the pc.
5. leaving the monitor off for a long period of time before turning it back on.
6. checking all the connections and cables.
7. switching out the monitor cable.