Phone not powering on, only red charging light.

Hi there,

I have just bought a HTC One M8 that had many issues. It would not power on and when I plugged it in, half of what looked like a battery icon and then a load of colors appeared on screen (the display was damaged) I opened it up and replaced: the battery, the SD card and volume flex, charging port flex and LCD display. I put it all back together with no problems but now when I plug it in, all I get is a red light and nothing on the display. I thought the display could be defective because nothing was coming up so I plugged in the old one and I still get a blank screen. I have searched everywhere and I don't know what could be wrong. I have heard that if you charge it with a quick charge charger for 10minutes and hold some buttons down, you can re calibrate the board and it may turn on but I don't have a quick charge charger and have tried pressing the buttons with my standard charger and have had no luck. I would like to know what other people think before I buy a charger. Any ideas what's wrong?