I have a MB C230 of 2007. A few days ago the steering got quite heavy.What could be the cause?

You could have low power steering fluid,. Check that first. If the level is ok then it could be a faulty power steering pump reservoir. There is a filter in the reservoir that can clog and cause a whining noise and hard steering. It can also be the pump itself but start with checking fluid. Alex

Low tire pressure - gauge tires.