AlexMurphy is right, check out that link. It very well could be (as the link states) that your power supply isn't providing enough current. Also, if you are running a pi 3, if my memory serves me, wifi is built in. You not giving us any base information to work with makes things difficult to help.
Also, if a better power supply doesn't work, I'm guessing that it's a hardware issue, not software, in which case, this is probably the wrong place to ask.