Your question doesn't give much information to work with. But most of the time when this problem arises suddenly, it's a dead or dying battery; you might have enough juice to light the lights, but not enough to turn the starter motor. Jump start the car properly; leave donor hooked up long enough to charge your battery, at least ten minutes. If your car starts, fine. If it does not start, replace your battery. After either of these, go to an auto parts store and have any codes read. They may point you to the reason your battery died. Again, this is just a guess culled from 60 years playing with cars.
New battery. And it's a 78, no computer in the car to check codes.
Sorry, when I originally saw your question, I misread it as 1998. With a new battery, this points to a bad starter/solenoid. If you turn the key and hear ONE click; bingo, replace the starter. If not, you have a problem in the circuit. Might be as simple as a blown fuse or voltage regulator, but you'll have to track down the problem.
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