Hi, lets try to cover the basics first.
Did you already verify your game files via Steam?
Do you have any launch parameters active in the launcher? If yes, try to remove them first, and try again.
Did you play on any modded servers before?
---
If this does not help you can try to:
Navigate in to C:\Program Files (x86)\Steam\userdata\*SteamID*\
Then there should be multiple folders, the ones relevant to DayZ are
221100 for stable
1024020 for experimental
delete both of them
Let me know how it goes 🙂