What it means

TomTom 'Battery too low to start' — or an empty-battery icon with the device refusing to boot — means the internal battery is too flat to power up.
On a deeply drained or aged battery the device can sit on the charger for a while before there's enough charge to even show the screen.
Often it's just a flat battery; sometimes it's a worn-out one, or a cable that powers the screen but isn't really charging.

Affected Models

TomTom GO, GO Discover, GO Supreme, GO Essential
TomTom Start and Via series
TomTom Rider
Older TomTom ONE, XL and XXL units (ageing batteries)

Common Causes

Device left unused for months and self-discharged completely flat
Charged from a low-power USB port that can't deliver enough current
Damaged or cheap USB cable that lights the screen but barely charges
In-car charger only powers the socket with the ignition on, and the car's been parked
Battery worn out and no longer holding charge

How to Fix It

Charge it for a couple of hours before expecting anything.

A deeply flat TomTom can sit with a blank or empty-icon screen for 15-30 minutes before it responds at all, and needs a few hours for a usable charge.
Use a proper mains USB charger (1 A or more) — not a low-power port on a laptop or a powered hub.
Try a different cable and charger.

Swap both the USB cable and the wall plug.
A tired cable is behind a lot of 'won't charge' cases.
A working charger should make the device show a charging indicator within a minute or two of being plugged in.
Check the in-car socket actually has power.

Many cars only power the 12V or USB socket when the ignition is on.
If you've been leaving the device plugged in with the key out, it never charged.
Test the socket with something else, or charge from the mains instead.
Force a restart while it's on charge.

With the device plugged in, hold the power button for about 20 seconds.
Once there's a little charge in it, that nudge often gets it to boot.
If it dies the moment you unplug it, the battery's worn out.

A device that runs only while connected has a dead battery.
TomTom batteries are replaceable on many models — DIY kits exist, or TomTom can do it.
On a very old unit, a new device may be the better call.

When to Call a Professional

If the device still won't power on after several hours on a known-good mains charger and cable, and a 20-second power-button hold does nothing, either the battery has failed completely or there's a charging-circuit fault.
TomTom support can advise on a battery replacement or repair — on many models a battery swap is a common, inexpensive fix and can be done DIY.