I replaced the battery and screen on my iPad and now it wont turn on.

I would make sure that the battery pads and the terminal all the board are fully aligned. Also make sure that the screw that holds the battery terminal down against the battery pads is securely fastened. To confirm if the iPad truly does not power on, I would connect to a computer. If the computer recognizes or asks for you to trust the connection yet you still see nothing on the iPad, you most likely have either a damaged LCD that will require replacement, or the backlight circuit has been damaged on the logic board and will require board level repair.