I cannot start my Laptop

Let’s give this a try, disconnect the battery again and with nothing connected press and hold the power button for a good 15secs to discharge the logic board fully, this will reset SMC if it was latched. Now plug in the MagSafe the system should spontaneously restart, let it full boot and do a proper shutdown.

If that fails your logic board needs repairs or replacement. You’ll need to pull it out to inspect it for damaged components. Post a few picks if you find a problem, one close and in focus one more so we can identify the area.