Hi darklyte, and welcome to AF
Probably the information which would help most is the history of the phone. Did you change/install anything shortly before it started (say few days)? Is it rooted? Have you had other problems, such as apps crashing or the phone freezing or restarting? Has the phone ever got wet (even months ago)? A bootloop can be due to a number of things, so trying to guess the cause might help identify the solution.
Generic advice: remove the battery, leave it out for several minutes - we want everything completely discharged, and ideally cooled down too. Then try again. If it works, hopefully you are OK but keep an eye on it.
If that doesn't work and you are rooted, go into recovery and wipe cache and dalvik.
If you are not rooted and the battery pull wasn't enough then we probably can't fix it without data loss. You can do this from recovery. Pull the battery out to force a shutdown, then reboot while keeping the "volume down" key pressed (press it the moment you start the boot and hold it). You'll get a white screen with a short menu. Use the volume keys to highlight "recovery" and press power to select. Will take you to a black screen with a red triangle. Once there, pressing volume up + power button should give you the recovery menu. There will be an option to factory reset or clear user storage or something like that (not erase SD card - that's different). That will remove contacts, messages, apps and app data, but there's not much else you can do with an unrooted phone.
If that's not enough we can look into reloading the phone software (which also erases user data, so there's no point trying this first).
If the phone has been water damaged at some point, or if the problem is due to overheating (e.g. if you've previously tended to suffer crashes when using the phone actively) there may not be much that we can do.
Sorry this is a bit of a brain dump, and if there's anything you are unhappy with please just post the phone history and see whether we have any ideas. I've posted this generic advice now because I don't expect to be on AF much in the next 24-48 hours, so outlining a few options now in case they help. The battery pull I'd definitely try, as there is nothing at all to lose from that.
If you are rooted don't try any factory resets without doing a full backup first (backup from recovery - not possible on unrooted phones with the HTC stock recovery, but rooted handsets should have a custom recovery with more options). Even if you have to wipe a rooted handset, Titanium Backup Pro may be able to recover data from a recovery backup.
If you don't know what "rooted" means, you probably aren't (though we've had some people buy second-hand phones which they didn't know were rooted. If recovery gives you a menu without messing with volume up + power to get it, then you are rooted after all).
Sorry, I should stop adding to the confusion now!