Few weeks ago I started having the same issue. If I turned my wifi off for any reason whatsoever, I wasn't able to turn it on again. I searched a bunch of forums with people trying to figure out how to solve that same problem, but no real solution...well... there is the obvious one, sending the device to guarantee. Unfortunatly, in my case, that wasn't an option, so I decided to start some experiments of my own.
I was able to find a workaround, it's not a solution since the problem will appear again given the same circunstances.
Keep in mind that this worked for me, but I cannot tell for sure if it will work for you. Anyway, if you have this problem and guarantee it's not a option I would give it a try before starting to think about opening the device or even replace the rom.
The workaround is really simple:
1 - Go to the Connections menu in Settings (where you turn on/off the wifi, bluetooth, etc);
2 - Turn on Flight mode;
3 - Turn on Wifi/Bluetooth (depending on your problem, apparently they both seem to happen);
4 - Turn off Flight mode.
And it's done. As I said before, this works for me and I can't guarantee that it WILL work for you.