What information does your router give you?
That means you’ve tried wired Ethernet as well as Wi-Fi?
The router might have given it a new IP Address, is your Wi-Fi password still the same?
The place to start is your router. Does the emonPi show in the list of connected devices in your router?Ethernet is the “safest”, because it should not need a password to connect.
The only reference I’ve got in my handy list is
Force the Emonpi to AP-mode - #4 by borpin which says:
To force an EmonPi back into AP mode, go into ‘WiFi’ from settings and deselect the currently selected WLAN. Click on save and then refresh. You will still have access to the emoncms instance. Navigate to the admin menu and click reboot. The EmonPi will now come up in AP mode (as no network is configured).
But if you can’t communicate with the emonPi in the first place, that’s not open to you.