Why won't my iphone charge or turn on?

I have had my iphone 4 for a year and 4 months. About 4-5 months ago I jailbroke it using greenpois0n, I never had problems from the jailbreaking that I know of. On new years I noticed it wouldn't charge with my phone case on the phone, so I would take the case off to charge it. I figured my friend messed up the charging dock on my iphone while using it to play music the night before. Over the next week or two I noticed the battery would die faster and faster. A couple nights ago, almost 2 weeks after it had the minor charging problems my phone died. I fell asleep with it charging while watching netflix. When I woke up the next morning I pulled the charger out and it was dead, it wouldn't charge or turn on. I used multiple USB chords and multiple outlets and computers. Nothing works, and computers don't recognize it when i turn it on. I want to know what is most likely wrong with it, if its fixable, where i can go to get it fixed, and approximately how much it may cost to get it fixed. Thanks so much!

Added (1). Yes i have tried holding the home and power buttons at the same time i've tried multiple different ways to reset it. It just won't turn on at all its completely unresponsive.

Curious… How does it turn on if it doesn't charge? And have you tried holding down the lock and home buttons at the same time for 5 seconds?
Possible battery failure?
Hmm

1.What's most likely wrong is that your battery reached the end of your life cycle and all the battery cells just lost there recharging capabilities (400) recharges. This could only happen two ways, one being you didn't treat your battery real properly by letting it die all the time, charging it once to many times or two you just got a faulty battery. Also yes this can be fixed. If you're a expert in taking apart iphones all you need is to buy a new battery -(http://www.ifixit.com/iPhone-Parts/iPhone-4-Replacement-Battery/IF182-000 or replace it with apple (http://support.apple.com/kb/index?page=servicefaq&geo=United_States&product=iphone)
2. If you have left your phone in the heat for way too long that could be a possible factor as heat damages your battery.
3. Other than that there's no other options i can think of:/

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