iPhone 4 will not turn on?!

I have an iPhone 4 from AT&T that is completely unresponsive. I have had this phone since about June 2011. It was working fine up until today. It had a full charge this morning, I had used it to text and last that I had checked it was at 90% before leaving to work. I got to work and hadn't touched it since I had used it to text my friend. I left it my locker at work and when I went on my break I found that it was off and totally unresponsive. I did not drop it or anything like that for it act this was. I tried turning it back on by holding down the lock button, directly charging it(left it to charge for two hours), holding down the lock button and home button at the same time for 20-30 seconds. Nothing!

What else can I do or might be the problem? 😕

Or is it just time for me to suck it up amd finally upgrade to the 5s?