you can solve this problem with three methods.
1. go wlan(wi-fi) settings select wifi connection which you want to connect enter right password..
after that go to ip settings and switch it from DHCP to static
after that enter the valid information (your ip,getway,prefix lenth
=24, Dns (Your Dns) ,Dns (same your getway) then save your settings.
switch off or reboot your phone. hope it will fix your issue.
or
2.turn off your device and turn it on with recorvery mode by pressing power nad volume + key same time.. then by pressing volume keys select clear emmc option for hard reset your device. clear delvik cache. clear cache also. after that reboot your device by using reboot option from recovery menu. try rebooting may be it'll fix your issue.
3. you can also fix this problem by updating your device from setting>about phone> system update(after update reboot your phone and wait for automatic restart.
(note. make sure you have a working internet connection with mobile data connection.)
you can also root your device using kingroot app after that install cwm recovery or fhiz touch recovery. and download fix wifi zip from xda devloper and flash it from recovery mode by using cwm or phiz recovery.