The lander, Schiaparelli, stopped sending a signal approximately 1 minute before landing. The ESA team is still analyzing data to determine what went wrong, but they expect something went wrong when the parachute was deployed. The lander likely crash landed on the Martian surface at a high rate of speed and was destroyed to the point of not being able to send a signal back. At a basic level the lander was a failure.
However, the mission itself is NOT a failure. Only a small amount of science was to be done with the lander. The lander was basically a test of the feasibility of landing future landers on the surface. The main goal of this mission lies in the orbiter, called the Trace Gas Orbiter. The orbiter has the goal of studying methane in the Martian atmosphere to look at the possibility of life on Mars at some point. Thus far, the orbiter is a success.
Getting to Mars is very difficult. NASA has had a lot of success in recent years, but historically, about half of all missions to land on Mars have failed. It's not easy launching something from one planet to another and a failure to land is by no means a mission failure. Again, this mission is NOT a failure.