So what happens when you go and try to turn the Wi-Fi back on? Does it just turn it right back off? Does it say that the Wi-Fi is on but you just have no internet? I might need a few more details to get to the actual issue and then a solution. If you are having issues connecting to Wi-Fi then check out Canít connect to Wi-Fi on an android phone? Hereís the fix
for some tips.
If the steps at the top of that article donít work then it could be a software bug and as a last resort I would perform the all powerful Master Reset (or Factory Data Reset). You will of course want to backup your data from your device first if you need help with that then look into reading How to Backup and Reset an Android Phone
If you do these things your phone should start connecting to Wi-Fi correctly again. Enjoy and donít forget to let us know how it turns out.