Your dashboard shows 'unavailable' for most devices, or the mobile app can't reach your instance. First, check if your server is actually running — SSH into your machine and run 'ha core info' to verify the supervisor status. If you're accessing remotely via Nabu Casa, log into their portal to confirm your subscription is active and the connection shows green. Local network issues often stem from router DHCP changes — reserve a static IP for your Home Assistant box in router settings. Firewall rules might block port 8123; temporarily disable the firewall to test, then add proper exceptions.
Devices won't connect even after entering correct credentials. Many smart home APIs changed their authentication methods recently. Delete the integration completely, restart the core, then re-add it fresh. For cloud-based services like Google or Alexa, regenerate API tokens in their developer consoles — old tokens expire. Check the integration's GitHub page for known auth issues with specific firmware versions. Some manufacturers rate-limit API calls; if you're polling sensors too frequently, increase the scan interval in configuration.yaml.
Your recorder database grew too large. Navigate to Settings > System > Repairs to check database size. Exclude noisy sensors from history by adding 'recorder: exclude:' entries in configuration.yaml. Purge old data with 'recorder: purge_keep_days: 7' instead of default 10. Some custom integrations poll excessively — monitor resource usage per integration in Settings > System > Repairs. Run 'ha jobs info' via SSH to identify stuck background tasks eating CPU cycles.