There are several reasons why your phone will not boot up. The first is a bad battery, the second and more popular reason is that your charging system is failing and the battery simply is not truly getting charged enough to boot the phone. However there is one other reason for all this. You mentioned your phone is rooted with a modified ROM. Sometimes the computer that is responsible for telling the phone how much charge the battery has gets confused and reports the wrong information to the phone. So, the phone thinks that the battery is to weak to boot the phone. Etradersupply.com helped me fix a similar issue by explaining to me about borrowing a friends fully charged battery. After installing it I was able to get in and reflash my ROM. Then once completed I restored my old battery and what do you know. It showed charged and the phone accurately registered the battery charge and so when I charged it it would take a full charge. Turns out it was a simple glitch that I might have been able to fix from the settings on the ROM if I could have booted the dang phone. There are other reasons that would cause this, some are due to hardware failure but it never hurts to try the simple fixes first.