iPhone 4, dropped and now won't restore (Error 21)

So, I have a jailbroken iPhone 4 (I know it's old but it's all I have) I dropped it, and it turned off. This has happened twice before, but both of those other times, it would go back to normal when I plugged it into a computer. On thursday, I dropped it, and when I got home and plugged it in, it was on a boot loop for SO LONG. It wouldn't stop. I didn't know what to do, so I stopped trying. Then, my dad helped me open it, and everything seems normal? So, I'm not really sure. It was still doing the boot loop, then I held down the home button while it rebooted and got the connect to iTunes logo. Everything was basically normal, I went to restore and I got error 21. I would appreciate any tips to help me PLEASE. Some people are saying the battery is no good which I guess would make sense, but, the battery had a good hold and was pretty strong, so I don't understand how dropping my phone (wasn't a far drop, either, no cracks) could make it die? Help PLEASE?