Diagnosis
The parent unit's battery has likely entered a deep discharge state, or the screen itself has a fault. Hearing the charge tone means it's getting power, but the system isn't booting up properly.
How to Fix It
Don't just plug it in for a few minutes. You need to perform a hard reset. 1) Plug the parent unit into the wall with its official charger. 2) Press and hold the power button (on the top) for a full 30 seconds, even if you don't see anything. 3) Release the button and leave it plugged in, untouched, for at least 4-6 hours. This gives the battery a chance to wake up from a zero charge. After that, try the power button again. It may take a few presses.
If That Didn't Work
While it's plugged in, try the physical reset pinhole on the back or bottom of the parent unit. Use a paperclip to press and hold that button for 15 seconds. You won't get visual feedback, but it forces a reboot. Then leave it charging for another few hours before trying to power it on.
Heads Up
To prevent this, try not to let the parent unit battery fully die. Charge it when it hits about 20%. If it goes black again after this fix, the internal battery is probably failing and needs replacement, which means contacting Motorola.