IPhone 4s stuck on set-up screen which leads to emergency calls only screen?

I haven't turned on my iPhone 4s in quite a while as I got a new one, but I'm trying to sell it and when I turn it on, it leads to the set-up screen that is a gray background that says iPhone on it, & the slider says "slide to set up" in a bunch of languages. However, when I slide it, it leads to the "emergency calls only" screen. How do I get into my iPhone?

1. Make sure a working SIM cars is inserted into the iPhone. In some countries such as the USA the iPhone is SIM-locked to a certain operator. In such case, make sure the SIM you are putting in it is compatible with the iPhone lock (meaning, don't insert a German T-Mobile SIM card into a device that's locked to American AT&T cards only)

2. Connect the iPhone while it's turned on to a computer with iTunes installed on it. If iTunes isn't opening on it's own when the iPhone is connected, just open it manually. After it has recognized the iPhone, you may exit it (don't do anything it offers you, such as updating your firmware version, you can do that later)

3. Press the Power button (top key) + Home button (the middle key at the bottom of the screen) together for 7 seconds until the screen goes black, then let go of the Home button but keep the Power button pushed until the screen lights up again.

4. After the iPhone turns on again, proceed with the setup as requested on screen. If you can, it would be preferable to connect to a Wi-Fi network during the setup when instructed to.

5. If it's working, good. In case it didn't, I'm afraid the "best" (or "only") solution I can offer is to restore the iPhone to a clean-installed firmware. You can do that with iTunes. Just make sure to back up everything on it first using either iTunes or iFunBox.