Mandeep,
Open the phone up and take the plate off over the lcd-digitizer/front camera flex cables. Do not disconnect the cables. Take the right screw out of the battery connector cover and loosen the left one so you can get a tool in to disconnect the battery. Reconnect the battery and try turn your phone back on and test the screen without reseating it or putting the lcd/digi cover back on.
If that works, make sure you have transferred the foam pads from the back of the connectors off the old screen to the new one and try re-assemble. It's possible that the plate is causing something to short out.
I've had this issue and ended up leaving the plate off so it would work as it was my personal phone and I didn't care.
If you do not have the foam pads use some electrical tape.