My iPhone 4s just randomly shut down?

I was just using it normally when the screen turned black and turned off out of no where. My lock button is broken so i can't turn it on manually so i tried to plug it into a charger which is how I would normally turn it back on but nothing is happening, the screen is still black and it won't turn back on.

It could be stuck in dfu (recovery) from the broken lock button activating it. There are only 2 ways to exit dfu mode. Plug it into your computer and do a full restore back to factory or hold lock button for 5 seconds, and without releasing hold the home button for 15 seconds then release the lock button and hold it until it comes on. (If it doesn't come on within 30 seconds try 15 seconds instead of 5 and 30 instead of 15 for the home button)

Without your lock button functional, the only option would be to plug it in as you did. If no battery or charging indicator appeared on your display, then there's probably another hardware issue inside the phone. Possibly your LCD or backlight. Turn your phone's switch up so it isn't on silent, and press volume up to be sure your volume is on. Assuming your lock slider is default and makes the unlocking click noise, you can mess around by trying to unlock it by sliding your finger as you would if it worked properly. If you can hear the functions, but the display isn't showing, it's your LCD. Also try holding your home button for a second or 2 and see if Siri will activate.