1. Verify DayZ game cache --Steam Library > right click DayZ select Properties > local files tab > Verify game cache
2. Check that you have the very latest video drivers for your card

If you don't know how to do these things use google.

You can clearly see the stars in the background because it's night time on the server you joined and that is the reason why you have a black/dark screen.

You can also see the silhouette of the nearby buildings etc.

Solution:
Join a daylight server.