I'll agree with Guest that your power unit is a likely problem with typical powered speakers. There's little that is likely to fail in the speakers themselves, but wires and connectors are common weak points. The wires used with speakers are often rather light weight, so the cat can chew a hole in one, and a little dirt or a bent contact in the jacks that things plug into can give an open circuit. (It seems to be nearly always the "female" half of connections that causes the problem, but I won't say that 'cause I'd have to run and hide. But "wiggling" things sometimes helps to identify where the problem is.)

If you can find the speakers you have been using at Dell Speakers it will give you and idea of what the direct replacement would cost. If you really want "comparable quality" it will give you an idea of what to look for.

Most computer stores should have suitable if not directly comparable speakers, if there's a good retail store in your area. It's common for speakers to outlast the computers they come with, so a "rebuilder" in your area might have something suitable on hand that might get better quality for what you'd pay at regular retail for a lesser set.

John