Dead battery. Maybe a bad generator/alternator not charging your bad, or not so bad battery? Perhaps some bad ground wire or something else draining your battery. Many possibilities. It takes more battery power to turn the motor over than to turn the lights on. Start with a jump & if that works, get your battery charged fully & take the car to a shop or parts store where they can test the charging system to see where the problem is. If you still have a generator, as I do, make sure & buy a replacement with a lifetime warranty. I have gone through 2 or 3 in the last 8 years & have not had to pay for anything except the 1st one.
Good luck
David K
63 comet convertible