Try going into the wifi configuration area, where the list of detected/available networks are, and tap your desired network, then tap "forget."

Next power down and back up, then set up the network again. This often remedies that issue.

 

Look in the list of WiFi networks when you want to connect...if you don't already.

Make sure the network does not have a lock on it. If it has a lock is it one you have connected to in the past? Remove the network and reconnect with the password entered carefully. Or maybe password was changed or the settings on router changed like channel frequency etc.

Troubleshooting connection issues require much information, there are so many things. Use my list suggestion to get started. Watch when you can't connect

 

I was having some trouble getting my phone to obtain an IP address on my home router. Then I got sick of it an reserved an IP on the router for it and set it to a static IP on the phone. Haven't had a single problem since.