Start by checking any and all wire connections first. That's always a source of issues on any vehicle. Including door wires and also behind the radio and amp (where applicable). Wiggle and operate ALL the radio buttons/knobs too in case there are connection issues with them (since they may not get used a lot) ...and be SURE the speaker settings didn't accidentally changed. Also locate the radio and amp fuses and check them VERY close. I've had internally corroded fuses that were really hard to see, which cuts the current needed for radios. Low current deprives IC chips of the 'right' amt of power and starts to goof up their logic. So good power is essential.
If all the speakers stopped working at once except the tweeters, that almost points to an electronic issue. But if some of the larger speakers are still working, then could be wiring or a connection issue, or partial amp issue. If none of the larger speakers work, then it's probably further up-stream, like a bad capacitor or other component (most always dried-out capacitors) affecting downstream power inside the amplifier which power the larger speakers. I'm just thinking out loud here.
If you are the handy type, I would track back to where the wires come from (amp or radio) and dig in a little, take it apart. More than likely you'll find a leaked out capacitor, or a bad solder joint at the power amp circuit board ...or maybe some darkening around the circuit board beneath the amp transistors (they might have burnt out). If you track it back to the amp section, begin the trouble-shooting process by inspecting all the electrolytic capacitors on the circuit board for leakage or domed tops (google these for pics). If you find an issue, replace them. It could just be one failed capacitor causing the problem. If they look good, then look for browning on the circuit board near the output transistors. If you find bowning around transistors, you know they were runninng hot, probably upstream power issues, or a transistor crapping out. You could try replacing a transistor (buy new parts off ebay), but at that point if you don't want to get into it, maybe just replace the radio, or amplifier (as appropriate).
OTOH, if the board looks 'clean' the next step would be to gently poke the output transistor's legs with a wooden chopstick (while the radio is hooked up and operating) and see if you hear anything ...like crackles or starting to work. Poke the rest of the circuit board and components here and there and listen for any changes. The poking would overcome possible poor solder joints which can often develop many years later ...be it TV's DVD players, etc. Poor solder joints are very common (called 'cold' solder joints). If you zero in on a cold solder joint, just re-solder it and you're probably good to go. Dried out capacitors are extremely common too and easily replaceable (new ones off ebay). If the tops are domed that's a sure sign of a failing capacitor. Even if it hasn't leaked, it's a sure symptom it's near death. Weak capacitors can screw up power to downstream circuits. There's ways to measure capacitor ESR (not your multimeter), but for your purposes, a lot can be accomplished just using your eyes, nose (burnt smells), touch (hot?) and ears (humming, buzzing etc). I knew a great troubleshooter who could zero in on electronics problems really quick just using the above senses. Heat is a leading electronics killer and being from Texas.
Of course, components might look perfectly good inside the amplifier and the problem could be a failed component (non visible) that would require deeper electronic troubeshooting which would be easier and cheaper to just replace the amplifier ..or radio (if no amp). But quite often problems can be detected with the troubleshooting techniques above.
Good luck!
