I have to post this for a friend:
I have the Yorkville xs400 Bass Head and a pick fell in through the grill on the top. Before I took it out I only got a really quiet fuzzy sound, but when I opened up the head and took out the pick I stopped getting any sound except on full volume and even then I only got a quiet farty noise. Any idea what might have happened?
"Everyone's life ends, but no one ever completes it."
Do you have an active bass? It's a big coincidence, but your bass's battery might have died at the same time you opened your amp. If not, I would suggest taking it to a music store. If you are at all unsure, let a professional do the work. You may end up doing more harm then good.

btw, by opening it up, you voided the warranty, if you had one, so don't expect any help from that.