my jeep won't start.

my dad just recently bought a jeep. when
he first bought it, the guy drove it out and it
worked fine. then it sat for a few months
and now it won't start. my dad said it's the
crank and can sensor that's wrong with it
and he bought the parts. any idea of what
it could be. he got it started two days ago
and now it just doesn't want to start. he
hasn't put parts in yet. suggestions?