It is difficult to understand what happened from the description.

1) Did the engine turn over?
2) Any clicking sound from the starter solenoid?
3) Did you cycle thru the gear shift in all gears and end in park again?
4) Did you try to start in Neutral.
5) Did you check the battery terminals and any small wires attached to the +ve terminal for oxidation?

If you hear clicking sound from the solenoid then it is possible that the starter solenoid contact is pitted. It does not have good contact to give the starter the current it needed.

Check 1-5 again. If it happens once then it probably will happen again. Good luck.

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300SDL project car