What it means

On TomTom models that store the map on a microSD card, 'No SD card detected' (or 'Please insert an SD card') means the device can't see the card — it's not seated properly, it's formatted in a way the device won't accept, it's failing, or it's the wrong type.
Without the card, those models have no map and can't navigate.
It's nearly always the card or the slot, not the device's main electronics.

Affected Models

TomTom Start models that use a microSD for the map
TomTom Via models with map-on-card storage
Older TomTom GO units that store the map on SD
TomTom Rider models with a memory card slot

Common Causes

Card not pushed fully into the slot
Card formatted as exFAT or NTFS when the device wants FAT32
Card failing or its filesystem corrupted
Card too large or too small for what the model supports
Counterfeit card that reports a capacity it doesn't really have
Dust or a bent contact in the card slot

How to Fix It

Reseat the card.

Power the device off, push the card in until it clicks and stays put, and power back on.
A card that's a fraction loose reads on and off, which the device reports as 'not detected'.
Check it in a computer.

Put the microSD in a card reader on a PC.
If the computer can't see it, the card has failed — replace it.
If the PC reads it fine, the device wants it formatted differently, or the slot needs a clean.
Format it FAT32, then reinstall the map.

Back up anything on the card first.
Format it as FAT32 — use a third-party formatter for cards over 32 GB, since Windows won't offer FAT32 above that.
Then reinstall the map through MyDrive Connect.
Use a card TomTom recommends.

The map needs space and a reliable card.
Check your model's supported capacity, stick to a reputable brand, and steer clear of suspiciously cheap high-capacity cards — those are often fakes that fail the moment the device tries to use them.
Clean the slot — then, if needed, contact TomTom.

A gentle puff of air clears dust from the slot.
If a known-good, correctly-formatted card still isn't detected after that, the card slot has failed — contact TomTom support.