There's not much advice I can offer. The first thing I'd try though is pulling the battery out of the phone and leaving it out for a couple of minutes (to ensure that the RAM is fully discharged). Then put it back in and connect to the charger again. I'd then leave it for a few hours to see whether it actually charges - if it doesn't then there won't be much more you can do (though if the battery has been damaged then a new battery would fix it, which would at least be a simple and inexpensive solution).
If pulling the battery doesn't fix it you could see whether you can get it into recovery mode: power on while pressing and holding the volume down key, and if it gets into the bootloader (white screen, short menu of options, probably some little android figures at the bottom) select "recovery" from the options (or if you can't see it, select hboot or fastboot, whichever is visible, then recovery). When in recovery (black screen with red triangle) you need some key combination to bring up the options - I think it's power+volume up, but I've spent very little time with stock HTC recoveries. If there is an option to wipe cache then try that, or in the limit you could try a factory reset (which will erase all your apps and data). To be honest though if it won't respond at all I doubt that will help (if it were partially booting then it might, but bad data will only be an issue when it's close to booting the operating system, it won't stop the boot from even starting).
If none of that works your best bet is probably to get it repaired or replaced. If it's 4.5 months old it should still be under warranty.
If you can tell us what exactly flashes when you try to turn it on that might help. But I really would try the battery pull (first suggestion) first.
Any clues as to what might have caused it, such as what you were doing when it froze, whether it has been dropped recently or whether it has ever got wet?