The door won't open and the washing machine is stuck with laundry inside — what's wrong?

The problem almost certainly lies with the door interlock. On more recent (and less budget) models, there is an emergency release lever, usually located near the filter, that allows you to open the door.

However, further checks will be needed to determine whether the door interlock needs replacing.

In some less common cases, the issue could be with the electronic control board, which is refusing to release the lock by not sending power to the interlock.

As we always say, it is best to call a professional engineer, because in a case like this the top panel will need to be removed to access the internal mechanism with a screwdriver.