We have proven that Mars used to have a magnetic field like Earth that protected it from solar radiation. We also know that it does not presently have a magnetic field. At some point in the distant past, Mars's core cooled and solidified, which removes the magnetic field.

The big problem is that the solar wind strips away the atmosphere and water, but that's (probably) not what killed all Martian life. As the magnetic field decreases, more and more harmful radiation reaches the surface. The planet was probably sterilized by radiation long before the atmosphere was lost and the oceans evaporated.

We're pretty sure this is what happened. We've been studying Mars's geology for a long time and we can see evidence for most of this process.