Three possible answers
1: Low battery.. Depending on how low you get click,click, (ratchating sound)
or a single click
2: Bad connection: My main engine does that when the battery terminals need cleaning, I hope I solved that one on a long term basis, but just now engine has other issues so.....
3: Starter lock,, In this situtation the gear the starter engages is often the culprit (though the starter gear can be) the gears engage wrong and can not turn due to being jammed.. SOMETIMES a wack with a rubber mallet will do it but usually you need to unbolt the beast (Starter) then wack it gently (NOTE always wack gently) and finally re-bolt it
It also means you are going to need to have someone look at it soon.