The Earth’s magnetic field deflects the solar wind from the planet and prevents it from bombarding the atmosphere. If the Earth’s magnetic field disappears for a long time, then the level of radiation on the planet’s surface would become mortally dangerous for most of the existing organisms.
This would result in the mass extinction of most species of animals and plants in a relatively short time, but most likely, in the long run, life could adapt to such conditions.
Besides, the solar wind would gradually blow away the atmosphere of the planet. This has happened to Mars in the past. Mars once had a very dense atmosphere and, apparently, oceans of liquid water on the surface, but after the loss of a magnetic field, the atmosphere began to gradually evaporate. However, this process is not fast and in the case of the Earth would go much slower than in the case of Mars. In order for the effect to be tangible, it will take several million years.
With the disappearance of the magnetic field for one day, we really would not have time to feel it, except that we would have seen auroras all over the planet, and not just at the poles. Besides, severe thunderstorms would have passed all over the planet, this is due to electromagnetic induction arising in an alternating magnetic field and the action of the solar wind as an ionizer of the atmosphere.
So far, we have talked about the effect on humans as a biological species. Indeed, if we were at the level of the Stone Age or even the Middle Ages, then the impact on this would be limited.
However, the disappearance of the magnetic field even for one day poses a tremendous threat to anthropogenic civilization. All our navigation aids will immediately collapse, which is fraught with the failure or incorrect operation of many satellites and is dangerous for all aircraft that will be in the sky.
The bombardment of the atmosphere by charged particles will lead to disruption of radio communications, which can also lead to destabilization of the work of many different organizations and structures.
Another problem is the failure of sensitive equipment, which can be damaged by the electromotive force resulting from electromagnetic induction.
Of course, this will not destroy human civilization, but the damage done to the economy and industry can become colossal, and it will take years, or maybe decades, to restore everything. With all this, the panic that will arise is a much greater danger, because of the lack of communication, states simply cannot control panicking people, and this is fraught with much greater damage to civilization.