What it means

TomTom 'Map corrupt' (or 'The map on your device is damaged') means the map files have become unreadable.
Almost always that's because a map update was interrupted — the device unplugged, or the laptop went to sleep, partway through.
It can also be a failing SD card on models that store the map there, or internal storage going bad.
The device can't navigate until the map is reinstalled.

Affected Models

TomTom GO, GO Discover, GO Supreme, GO Essential
TomTom Start and Via series
TomTom Rider
Older TomTom units that store the map on a microSD card

Common Causes

Device unplugged or computer slept during a map update
SD card (on models that keep the map there) failing or pulled out mid-write
Internal storage developing bad sectors
An interrupted file copy through MyDrive Connect
Device software that also needs reinstalling

How to Fix It

Reconnect to MyDrive Connect.

Plug the device into a computer running MyDrive Connect and log in.
It usually detects the damaged map straight away and offers to reinstall it — let it.
Keep the device connected and the computer awake for the whole download.
Delete and reinstall the map.

If it doesn't offer automatically, in MyDrive Connect remove the map from the device, then download and install it fresh.
A clean reinstall fixes the large majority of 'map corrupt' cases.
Test the SD card, if your model uses one.

On TomTom units that store the map on a microSD, put the card in a PC.
If the computer struggles to read it, replace it with a TomTom-recommended card — the map needs a fair amount of space and a card it can trust.
Reinstall the device software too.

If reinstalling the map alone doesn't clear it, MyDrive Connect can also reinstall the device application.
Do that first, then the map.
Factory reset as a last resort.

On the device, Settings > Reset (or hold the power button ~20 seconds on older units) to wipe it, then set it up again through MyDrive Connect.
Back up any custom POIs and favourites first if the device still lets you.
If a full reset and reinstall still won't take, the internal memory has failed — contact TomTom.