my punto 60sx would not start this morning.

engine kept turning but failed to start.

it was fine last night.

i was going to call the AA but i waited 5 minutes,then used the spare key and it started ok.

thereafter, i used the original key with which it failed to start this morning and it works fine.

what could have caused this?

If it happens again look at the code light and see if it goes out 1/2 sec after you switch on.
Does it go out?
Yes = engine area fault, maybe crankshaft sensor going duff.
No = key/immobiliser problem.

Sometimes keys can do this its known i think the key can loss its code thats why you should allways have a spare..