From FAQ, Answer 000
= [Q:] I cannot connect to My Arduino Yún. What do I do? =
First it’s good to know, when Arduino Yun cannot find the AP you have set it for (like your home wifi), it has a script that puts it back into default SSID and AP mode.
Next, you may have already watched this video, but it’s worth watching again. Pay extra attention to the part with the sketches.
Getting started with Arduino Yún - tutorial (6:53)
Then, read the short instructions immediately below. We (the volunteer support group) already know these steps by heart, so be familiar with the different reboot methods, and especially the 30+ second wifi reset. Also, not mentioned on that page, if you reset the Wifi/Linux part of the Yun, it will take from 2-8 minutes for the reboot. Lastly on this part, YOU MUST WAIT for the Linux portion to fully boot BEFORE YOU PRESS ANY of the RESET buttons.
Resetting the processors (AR9331, WiFi, and 32U4)
Then start over, plug your Yun into the USB port, run this sketch, and hit the ‘‘Wifi Reset button’’ (for 5-10 seconds). (see attached image)
The YunSerialTerminal provides a simplified console access to the Linux part of the Yun. It works via the USB port (the one that powers the Yun - the microUSB). From there you’ll be able to run a few commands to give us a better idea of problems.
Also, be ready with this sketch YunWiFiStatus. This sketch will help us see the status of your home AP. The sketch prints the wifi status to the console every 5 seconds or so. This sketch might seem familiar as it was in that first video link (above).
Let us know how it goes.
Post results from the YunSerialTerminal, if you are still having problems.
Please use markup when posting results. (see attached image)