BB Bold :: 9900 Showing Red X?

I have a Blackberry Bold 9900. I haven't used it much for about three months and when I tried and turned it on, it shows me the red cross on a battery. Initially I thought the problem lies with the batteries as they hadn't been charged for a few months. I tried different methods to bring it back to life and by luck, I managed to charge the phone and it worked for 2 days.

Then I tried to restart my phone. It had almost full battery then. It turned off and then the red cross on the battery came up again. So I thought it might be a problem with the software so I made the stupidest mistake by connecting it to my computer and trying to repair the software.

Now my Blackberry is not turning on and constantly shows me the red cross. When I do manage to charge it, it gives me error 507. And when i try to connect it to my computer and repair it, the red cross comes up when the desktop software is "waiting for device to initialise". I've had this phone for about 9 months and it worked fine until I neglected it for some months.