"iPhone won't restore" is a common issue. However, there still are some ways to fix this issue. Follow below instruction to have a try.
1. Hard reset iPhone
Hard resetting your iPhone is not highly recommended, but it indeed works sometimes when your iPhone won't restore or gets stuck. Make sure your iPhone is plugged into your computer while performing the hard reset. And it should be noted that the hard resetting differs upon your models of iPhone.
2. Try to restore your iPhone in DFU mode
DFU stands for Device Firmware Update, and it's the deepest type of restore you can do on an iPhone. It's time to try a DFU restore if, after trying all the methods mentioned above, your iPhone still won't restore. This is a special type of restore that clears your iPhone's hardware and software settings, giving your iPhone a completely clean slate. Oftentimes a DFU restore will let you restore iPhones that are experiencing software difficulties that prevent normal restores.
3. Update iOS to the latest version
Check whether your iPhone has been running on the latest iOS. It's easy to do the check work: on your iPhone, open "Settings" app and then go to "General" option. Open the "Software Update" option on the "General" tab. Your iPhone will check if there is any iOS update available. Follow the on-screen notifications to update your iOS if an update is available.
4. Put your iPhone into recovery mode and set it up again
Sometimes your iPhone won't restore because iTunes doesn't recognize your iPhone, or your screen is stuck on the Apple logo for several minutes with no progress bar. In such situations, you could try to put your iPhone into recovery mode and set it up again to fix "iPhone won't restore" issue. The detailed operation will be discussed below.
If above methods don't work, check this post: 5 Methods to Fix "iPhone Won't Restore" Issue