DIAGNOSIS: The battery is critically low and the charger isn't establishing a connection. The blinking red light means it's trying to charge but failing.
FIX: Clean the metal charging contacts on both the handle and the charging base. Use a dry cotton swab or a pencil eraser to scrub off any toothpaste residue or corrosion. Then, press the handle firmly onto the base until you feel it click into place and leave it for at least 24 hours. Don't try to turn it on during this time.
IF THAT DIDN'T WORK: The battery may have degraded from being stored completely dead for too long. Try a different power outlet and ensure you're using the original Oral-B charger. If it still blinks red after 24+ hours, the internal battery is likely dead and needs replacement.
HEADS UP: Never store this brush with a dead battery. If you're traveling, give it a full charge first. A fully drained lithium-ion battery can permanently lose capacity.