At the beginning I couldn't find it too,
you can try this app if you have android phone.
Fing - Network Tools
This app scans all your network and shows the nodes
If you cant connect via wifi try connecting with Ethernet and then configure the Yun.
Or you can totally reset the Yun.
In order to reset to factory settings push the WLAN RST button for 30 secs.
If you reset the Yun all settings will be lost