quote]Anyway, this was working fine last night.
This evening, however, I booted up and had lost both sound and internet.[/quote]
I'd have been inclined to check that it functioned at a later date as the problem could have been with the ISP (or does your phone still work? You hadn't hit a data threshold with the ISP, had you? Does your Windows PC still work with the tether? Did you power everything on in the same order as when it worked?
You haven't mentioned any of these things. If your car doesn't work, it's easier to check for lack of fuel before stripping your engine!