Several possibilities, in order of cost and difficulty:
Shift the transmission to D and back to N or P to test the neutral
safety switch. OK, that was a dream, on to the battery.
If the battery is > 4 years old it's suspect. My M-B battery failed
within one week of 4 years, so don't dismiss it or....
A new starter is needed. To determine that possibility, the other two
possibilities have already been eliminated, so hit the
starter motor with a 2 x 4 whenever it fails to turn - eventually that
won't work either but you'll know that the starter is the culprit!