Laptop Sound Not Working after using it with HDTV

I've a Dell Inspiron 1545 Laptop. Today I connected it with my HDTV to watch YouTube Videos.
But when I disconnected it, the laptop speakers sound was not working.
I'm using Windows Vista. It was working absolutely fine before connecting it to the HDTV.
In the Task bar, the speaker icon is looking like this.

What happened to the sound card.
