Yun forgetting wireless settings

I have been using a yun for some time. It is set up to connect to another yun which acts as a wireless router. This has worked well until recently when the yun seems no longer to remember that is supposed to connect to the other yun on start up. It appears as it's own wireless router. I can restore the configuration but when the system is powered down, it doesn't, remember when it next restarts.

Any ideas what may have happened. I have not knowingly changed any settings.

Hopefully,
NickH

When the yun boots, it tries fails to connect to your router, after 60 seconds, it will reboot and get back to Access Point mode
When this happens and you access the webpanel, you should see an error message that explains why the yun has failed.
If you then just reboot the yun (unplug & replug), it should retry the last configuration (the one of your router)