Can you elaborate more on what exactly is happening?
Did it work before?
What exactly happens when you open the Contacts app?
Are your contacts synced to any service such as GMail or iCloud?
Have you tried a hard reboot? Press the Home and Power buttons at the same time and HOLD them until you see the Apple logo.
Again, what happens when you try? What specific app are you asking about?
What troubleshooting have you tried?
Are you saying that the stock "Contacts" app is not showing on your phone? Does it show up in the Spotlight Search? Did you accidentally put it in a folder? Did you delete it? Have you tried reinstalling it from the App Store?
Are you saying that the contacts IN the app are no longer there?
Make sure contacts is toggled on in settlngs > iCloud > Contacts. If it is, toggle it off, choose delete from my phone, then toggle back on. If they are Google contacts, do the same in settings > mail > gmail. If you have Facebook contacts, go to settings > Facebook > Toggle contacts on. Then tap "update contacts" in blue at the bottom.