I TRIED THE FOLLOWING ON MY PHONE AND IT WORKED... 100 % I HAVE HAD MY GALAXY S4 FOR A LITTLE OF OVER TWO YEARS WITH NO ISSUES AT ALL, RECENTLY MY POWER BUTTON SHORTED OUT AND HAD TO HAVE IT REPLACED, OUT OF WARRANTY I WENT TO A PHONE REPAIR PLACE. GOT IT FIXED ABOUT ABOUT 2 MONTHS AGO. AND NOW JUST STARTING TO HAVE THIS ISSUE WITH THE WIFI AND BLUETOOTH NOT WANTING TO TURN ON. NOT BLAMING THE REPAIR PLACE,SINCE I HAVE DROPPED MY PHONE MANY TIMES AFTER, SO I TRIED THE BELOW AND IT NOW IT WORKS FINE. I BELIEVE THIS IS A HARDWARE ISSUE
*~DISCLAIMER -- BE GENTLE and USE CAUTION when trying this next suggestion, you do NOT want to cause any damage to your phone and make your current situation worse~* --I will not be held accountable for any damage done to the phone when trying my next suggestion--.
Gently, and carefully give your phone a good shake, don’t let it slip out of your hand, fall to the floor and cause its screen to shatter. You can also give it a few gentle love taps on the back of the phone, (don’t hit and break the screen, I recommend tapping the back of the phone).
After tapping or shaking the phone, test the Wi-Fi and see if it started functioning properly. It may take a few attempts to get a lose cable or piece of hardware back into its proper place. So if at first you don’t succeed you may need to try a few more times.
If it starts working again, then you have narrowed it down to a hardware issue and you’re now faced with a few options…
You can keep using the phone as the piece of hardware may stay where it’s supposed to and you can keep enjoying your working Wi-Fi.
You can keep using the phone and if the Wi-Fi acts up again you can give it another shake or tap to get it to work properly again. Just be careful as this might get frustrating if it starts happening frequently and you don’t want to accidently damage your phone by hitting or not so gently tapping it against a hard surface out of frustration.
You can keep using the phone until the Wi-Fi acts up again and then file a warranty claim. If you are no longer under warranty you can consider taking the phone apart to locate and try to fix the problematic hardware, which may be as simple as pressing a wire or cable back into place, in which case you will have a more permanent solution to your Wi-Fi problem.
Software
If it doesn’t seem to work after a little shaking or tapping then this could be a software related issue.