What to write when you don't know what to write

Has it ever happened to you that you decide to start writing and you don't know what to write? It's not that you're out of topics, but your mind is spinning with a bunch of them and you can't seem to choose one. Finally, you settle on a topic, but as soon as you start writing, you find yourself in a state where you can't write a single line. At that moment, you decide to write about another topic, but you can't write that one either, and the cycle repeats with the next ones. Sometimes you even start alternating between topics, selecting and rejecting them again and again. This mental process continues and you don't write a single line.