I was having this issue. Now, I haven't changed any router settings or iPhone settings... so I refused to accept that I needed to clear out my settings. I was willing to clear out my settings if NOTHING else worked.. but anyway I tried everything I could think of that didn't involve resetting all my settings. I rebooted my phone, I turned on airplane mode, I carefully entered my password a dozen times or more, I rebooted my router, I tried manually entering the SSID and security mode. Nothing worked.
Then, I was reading the posts where some people changed their password on their router and then changed it back the the iPhone connected both times. So I thought to myself, "What if I do it the other way around?" I had been entering the correct password on my iPhone when trying to connect to my SSID - and getting an error that it's wrong - "What happens if I actually enter a wrong password?" Crazy idea, I know, but I did it.. and it worked for me:
1) Intentionally entered wrong password for my wifi network when trying to connect
2) Received error (obviously)
3) Then tried correct password again
4) Connected successfully.
I agree that I have had more problems with my Apple stuff than I care to mention lately and I am no longer an Apple fan.