I'm suddenly having the same problem. It can't even see available wifi - let alone download new purchases and sync with my tablet. REALLY annoying. I've tried manually - but I seem to have the most up-to-date software. Any other suggestions gratefully received.
Maybe you are. Have read this thread which has solutions, or just having a moan.
As only you know what device are using and what software is loaded on it the above is a stab in the dark.
All of a sudden my Kindle started dropping the wifi connection at random times. So I followed the advice given here: I rebooted my Kindle by holding down the power button for 30+ seconds. And....VOILA! No more dropped connections! Simple solution, but it worked!