My Gecko Is Not Eating What Should I Do?

It's hard to give an answer with so little information...

What kind of Gecko is it?

How long has it been since it's eaten?

What are you feeding?

What kind of set-up and temperature is it being kept in?

How long have you had him?

All I can say.. Is make sure the temp is up... And if it's a tropical species.. That the humidity is kept high.  Try a different live food source.. Sometimes a change of menu will tempt a finicky appetite... But 9 times out of ten, when a rep. Goes off food, it has to do with habitat errors (assuming it's been healthy in captivity for a while now)..