A bad idle air control valve can cause hard starting or a failure of your vehicles engine to start.
The idle air control valve allows enough air into the vehicles engine for it to start and if the idle air control valve is not working it will not allow the air to get into the engine which can cause either no starting or hard starting.
A bad idle air control valve can also cause an irregular idle speed of your vehicles engine so your vehicles engine does not maintain the proper idle speed.
Other things that can cause a vehicle to be hard to start are, fouled spark plugs, failing ignition coils, dirty or clogged fuel filter, dirty or clogged fuel injectors etc.