PLEASE READ EVERYTHING.
Okay, so what happened in my case was that I downloaded AppSync and AppCake. I think it was the AppSync that put it in a boot loop. When it was in a boot loop, I tried doing the Hold Volume+Home+Volume Up button thing and it didn't work. So this is how I fixed it, from what I remember. This is my first tutorial so I hope I'm explaining it correctly.
What you need
A jailbroken iPhone that's on a boot loop not starting up.
Your iCloud account that's logged onto the iPhone, security questions & maybe your phone number.
Location Services on
Also, put your iPhone on the charger.
Steps
1. Make sure your phone is on.
2. Go to https://www.icloud.com/ and sign into your iCloud account and go to Find Your iPhone.
3. Click "All Devices" and click the device that's on the bootloop.
4. When you click on the device, click Erase Phone.
5. Click Erase.
6. You're gonna have to enter in your iCloud information (make sure that's the iCloud info that's on the iPhone)
7. After that, you're going to end up answering your security questions and entering your phone number. (The phone number part happened to me. Not sure if it would happen for you guys, but it might)
8. It should pop up something that says it'll send you a message to the iPhone. Just click Done and wait.
9. Your iPhone should doing it's thing after that.
The steps is based on what I remember doing. It worked for me. Now after you finished resetting your phone and putting on the jailbreak, Cydia would give you the var/lib/dpkg error or something.
This video helped me out with that problem.
"How to Fix "var/lib/dpkg.." Database Error & Fix Cydia Crashing on iOS 10 - 10.2 Jailbreak!"
By Brandon Butch
If I should fix anything, let me know. This is from what I remember. I didn't get any errors or anything and the video above should help you with any problems with Cydia. If I get any errors or problems I will update you guys on it.