Both leds stay constantly on. So fault must be in sd card. I formatted the sd cards in full and copied the files as it is done. Did this for a new sd card. I tried sdformatter and windows' own formatting tool. But it just doesn't boot up.
Both leds stay constantly on.
This is exactly the same thing that will happen if you plug it in with no SD card inserted. Try it.
The card is formatted incorrectly.
The SD card is one the reader doesn't like; this was more an issue with the original models, but I believe people have had problems with certain 64 GB cards on the microSD models (+/2/3/0). Try a completely different card.
The Pi is broken.
Ruling out #2 is easy unless you are in a strange remote land where only one make and model of SD card is available, so that really leaves #1 and #3. Unless you have another Pi, you can't even infer much about #3, so which one you want to pick depends on how tenacious you want to be focussing on #1.