Take it to a mechanic (dealer or independent or friend in your local Miata club) and have them interface with OBDII and find out what fault codes have been tripped. Then have them take that information, flip open the factory workshop manual, and go through the detailed diagnostic procedure to find out what's really wrong. My 2001 manual has a few hundred pages dedicated to the engine management system and troubleshooting. You might as well start with the cheap and easy DIY repairs but it could be anything from bad gas to ignition coil failure to O2 sensor failure to who knows? It can't be properly diagnosed over the internet but if it started up and ran fine today I'd probably take the risk of driving it (as opposed to having it towed) to said mechanic for inspection, diagnosis, and repair. There's nothing magic about 60k miles since so many factors determine wear and tear / component service life. Now's a good time to have all that routine maintenance done like timing belt, check valve clearance, accessory drive belts, engine and tranny and diff fluid change, air filter, spark plugs, plug wires, PCV valve, fuel filter, clean throttle body/intake manifold, etc. Good luck!