Based upon the study of lava flows of basalt throughout the world, it has been proposed that the Earth's magnetic field reverses at intervals, ranging from tens of thousands to many millions of years, with an average interval of approximately 300,000 years. However, the last such event, called the Brunhes-Matuyama reversal, is theorized to have occurred some 780,000 years ago. So as you can see, we are  little overdue. Right now, as I type this, there are 11 poles on the planet. Each pole point is a weakness in the magnetic field. At these points, radiation from the Universe can get through. With ease. There is no clear theory as to how the geomagnetic reversals might have occurred. Some scientists have produced models for the core of the Earth wherein the magnetic field is only quasi-stable and the poles can spontaneously migrate from one orientation to the other over the course of a few hundred to a few thousand years. Other scientists propose that the geodynamo first turns itself off, either spontaneously or through some external action like a comet impact, and then restarts itself with the magnetic "North" pole pointing either North or South. External events are not likely to be routine causes of magnetic field reversals due to the lack of a correlation between the age of impact craters and the timing of reversals. Regardless of the cause, when magnetic "North" reappears in the opposite direction this is a reversal, whereas turning off and returning in the same direction is called a geomagnetic excursion.

If our planets geodynamo did turn off, that would be the end of life on Earth as we know it. There would be nothing to stop radiation from the rest of the Universe.

The Earths magnetic field has changed on so many occasions due to the Earth itself. Fault movements of tectonic plates have caused this on numerous occasions. Let us not forget that it is the magnetic field that stabilizes our atmosphere and water levels. The Earth's magnetic field strength was measured by Carl Friedrich Gauss in 1835 and has been repeatedly measured since then, showing a relative decay of about 10% over the last 150 years.