The airplanes weren't able to fly, because it could damage the engines of the airplane, it can weaken the visibility and the cloud can get into the cabin and that is really dangerous for you.
These were really troublesome weeks for airlines. They lost almost 2 million euros per day and that's a lot!
There are people who benefitted of this. Trains, busses, car rental companies all were really happy, because everybody used these vehicles. The car rental companies knew this that people would rent cars so they raised the prices. For example my sister's husband had to drive from Greece to London, because there was no way he could fly home.
Now this chaos has settled and the air traffic is almost back to normal (atleast for now).