If the battery has run completely flat and still won’t charge after trying a hard reset all you need to do is contact TomTom support via their website.
The same thing happened to my Tomtom & once they had registered it as a return to base repair they sent out all the necessary packaging to return it safely & they repaired it and returned it to me free of charge.

I reckon it’s battery.
After a full mains charge (24 hours solid), it loses 25% overnight WITHOUT being switched on!
Present owner can mend it, the battery is glued to the PCB I believe.