As a diagnostic/troubleshooting aid, have you tried installing and booting from a standard Raspberry Pi Foundation release of their Raspbian Operating System?
the rpi booted before.
So what has changed? Did something happen to the RPi that could have caused it to develop a fault?
Edit: added later, to try to prevent this dragging on in frustration -
There is a Troubleshooting guide in Issue 60 of the MagPi magazine which you can access for free online. https://www.raspberrypi.org/magpi/issues/60/
The Raspberry Pi Foundation have a Troubleshooting Guide on their website: https://www.raspberrypi.org/learning/tr ... ing-guide/
These forums have a post which contains all the information currently available about getting an RPi to boot: "STICKY: Is your Pi not booting? (The Boot Problems Sticky)" at viewtopic.php?f=28&t=58151
The key to all those information sources is that only 3 vital components are needed to boot an RPi board into a running Operating System: an adequate and reliable Power Supply; a correctly written microSD card; and an RPi board in working order. (A display is only optional because an RPi can be used 'headless'.)
If you are certain
, as you appear to say, that your Power Supply and microSD card are not at fault the only logical conclusion must be that your RPi board is faulty.