Suddenly battery problem on Dyson v15

The battery has a fairly smart BMS board. When the pin is depressed, it likely intentionally disconnects the main power rails while the Cricut checks to an analog front end for the microcontroller to check for fault states like over-discharge, low voltage, overcharge, high voltage, short circuit, cell SOC balance, and operating temperature. This is a safety feature and an expected behavior. The battery that doesn't restore itself to operating voltage likely failed the battery health check for one reason or another.

I'd check the cell balance and cell resistance individually I'd you are knowledgeable and can do so safely.

I'd also measure the BMS's temperature sensor in resistance mode on your multimeter, and check that the value is close to 10k ohms

Check the handle's battery terminals for a short circuit

Ensure the power switch electrically closes.

Clean or replace all filters and empty the dust bin

Open the vacuum and ensure the blower motor isn't stuck and there is no debris lodged inside.

MyCheck recommendation is to replace the battery pack with an OEM one.

I would not trust an AliExpress power tool battery to work properly for something as finicky and proprietary as a Dyson hand vac.

Please always follow safety guidelines for handling lithium-ion batteries and disposing of them properly.