Suddenly I get no audio from left speaker when playing CD's or DVD's. Playing sounds from Control panel or PC Doctor also doesn't play from left speaker. Has my sound card gone bad or should I check something else?

in volume control has the balance been moved across

How do I find that? I think I've checked everything. Is that from Control Panel-->Sounds and Audio Devices ?

It suddenly started working again today. I didn't do anything. I rebooted this morning and now it works. I don't think it was the rebooting that did it, because I rebooted several times yesterday with no effect.

check your connections, there may be a loose connection somewhere