Try the following as a temp solution:
1. Put Airplane Mode on
2. Turn off device
3. Wait few seconds and turn in on
4. Make sure bluetooth is off
5. You should be able to turn WIFI on
6. Turn Airplane mode off.
This works for me but I'm taking my handset to apple store as this issue happens to me when I upgraded to 6.1.2. They say it is a hardware failure. Even if it is the underlined cause of this is software issue.