After seeing your post, I decided to check my Kindle. I had no problem connecting via WiFi and downloading a book.
I'm fairly new to this also, but try going to menu and checking if the wireless is on/off. Try to change that and maybe it will turn on again?
Also, on the home page, in the uper right hand corner, do you have the little (Wi-Fi) symbol, and the bars of strength next to it?