Hey Zuhaib,
First off, this may sound stupid but are you sure the address you are trying to get directions for is a valid address?
If it is valid, then there is a few more things you could do to try and resolve this:
1.) Restart your iPhone by holding the Sleep/Wake Button and the Home Button together until the screen goes black and the white Apple logo shows up (I see you have already tried this but sometimes a second try can't hurt!)
2.) Double click the Home Button on your iPhone and it will open up the multitasking bar/menu, from there swipe up on the Maps app to completely remove it from your multitasking bar, then open it up again- this sometimes fixes my problems. (It essentially restarts the application).
If this doesn't work let me know and I will try and help you out some more.
Regards,
NHS