A common problem is the connectors onto the starter solenoid. There is a small wire with a spade connector (which comes from the relay box) on the top of the solenoid. Take it off, clean it up, and put it back. Then try it again.
If not, as others have said, listen to the starter as someone else presses the button. If there is no click from the solenoid, you may have a dead relay.
If the solenoid is clicking, then the starter motor might be jammed, and quite often a good wack with a hammer on the casing is enough to get it going again.