1. Ensure that your WiFi router is on and running. Check the lights on the router to ensure it's functioning properly.
2. Reboot the router. Unplug the router from it's powersource, wait about 10 seconds, then plug it back in and wait for it to fully restart.
3. If you're still unable to connect, continue following the steps below.

Fix 1: Turn Wi-Fi off and on.

1. Go to Settings -> Wi-Fi.

2. Toggle Wi-Fi Off, wait a few seconds, then turn Wi-Fi back on.
3. If your Network is still not appearing, continue to Fix 2.

Fix 2: Restart your iPhone.

1. Press and hold the volume down button and the power button until the power slider appears.
2. Drag the slider to turn off your iPhone.

3. After the device turns off, wait a few seconds, then press and hold the power button until the device turns back on.

Fix 3: Reset Network Settings

1. Go to Settings -> General -> Transfer or Reset iPhone -> Reset
2. On the popup, click Reset Network Settings.

3. Enter your password if prompted and confirm. Your iPhone will restart.

1. If your network is hidden, (SSID is not broadcased), follow these steps.
2. Go to Settings -> Wi-Fi.
3. Click on Other...

4. Enter your Network Name, Secutiy Method, and Password, then click Join.
5. If the details entered are correct, your iPhone should now be connected to the Wi-Fi.

See Also: 

How do I Connect to a Wi-Fi Network?

iPhone keeps disconnecting from WiFi