On Steam...  Right Click Ark and select Properties
Then click "Installed Files" and "Verify Integrity of game files..." Integrity of game files..."

On EPIC  

Click the three dots underneath Ark and choose "Manage"
Then click "VERIFY"

On Windows, it's worth doing a system check regardless of the outcome of the above.
In an Administrator command prompt, run   sfc /scannow

If the above command says it found errors that it couldn't fix, then run the following commands...

DISM /Online /Cleanup-Image /CheckHealth
DISM /Online /Cleanup-Image /ScanHealth
DISM /Online /Cleanup-Image /RestoreHealth