First of all try this
Unplug the modem,
wait 30 seconds,
plug it back in,
go to the iPhone settings and select general and then network settings and Reset the network settings,
Restart the iPhone and try to connect again, putting in your password when asked
If this does not work then we'll need some more information like
1) What else uses your home network?
2) What encryption do you have 64 bit / 128 bit etc
3) What have you tried (for ruling out purposes)
4) What router do you have?
5) Go to iphone settings, wi-fi, select the blue arrow next to your network, what does each section say?