My laptop is on but doesn't show display
This is a common issue that you can fix by following these instructions:
Open the Apple menu. Select System Preferences. Select Displays and choose your laptop's name. In the right-hand pane, adjust your screen resolution to a low setting (800x600 or 800x854). Reapply changes and restart. ** This issue may occur if your MacBook Pro has a video card installed but no monitor connected, or if it was removed during repair process.