Beep sound and LED on mb blinked

Okay i had an itchy finger and opened my chasis side panel to have something removed from the chasis. The computer was on that time. And then suddenly accidently my finger touched the fan on the cpu cooler. The fan is connected to the cpu fan header on the mb. If not mistaken the fan stopped a while and continue to resume shortly after. Then there was a bios beep and the LED to indicate my computer is on blinked. What happened and why did the alert beep when off? I only touched and stopped the fan a second. Even if the fan is not running the mb shouldn't know right? The question now is why the LED blinked and an alert beep sounded?