Disoriented and stupid

Today, I should have gone to a lecture by Mr. Fejfar Physics in biology.
It should have been in a Matfyz building at Karlov. I even found the building.
Yet I didn't recall the number of the classroom. I simply hadn't considered I might have needed it. Stupid!!!
The gatekeeper didn't know anything about the lecture. I even tried looking around the building, but found nothing. Tough luck.
When I returned home, I got to know it should have been somewhere in the basement. How should I know there's a basement?
I just fear, if I study there, how will I be finding the classrooms...