Hass.IO on Pi3 - availability issue

Installed Hass.io on Pi3 per the instructions. Everything is working fine till few hours after the UI not accessible anymore. I have tried to access via internal IP or hasslocal. same problem. I know the unit is on and hass is running since I can SSH into it and get hass logs. I don’t see any problem from the logs and able to resolve after restart. Any clue what could be the problem?