Try and turn the Impreza engine over by hand. The engine could be locked up and not let the starter turn it over.
It will probably be your starter motor Starter is out!
there is no starter relay have your car alarm checked it has a starter disconnect that goes bad
on the righthand side of the the gearbox behind the engine.
When the starter motor refuses to turn over or grinds and clicks when turning then you need a new motor or recondition the old one.
if the battery and battery conections are good, then you need to replace the starter motor
bad starter motor
If it is a v6 motor left side of trans on starter if it clicks when trying to start sometimes you can peck on starter motor to get started but will stop working again will need replaced
Typically this signifies that your starter motor has either jammed, or is faulty. To check that it has not jammed, give it a whack on the end with a small hammer, or fairly large wrench. If that does the trick, then the starter motor was jammed, if not, you need a replacement starter motor.
Your battery is low. You probably has to replace the battery. Or maybe the starter motor solonoid is faulty
Either Discharged battery or poor/loose battery or starter connections or solenoid contact need replacing
The single click, that you hear when the starter switch is operated, is the solenoid making the high tension connection to the starter motor. If the starter motor doesn't turn over, you will need to check both the battery feed to the solenoid and the feed from the solenoid to the starter motor (this is typically a short length of braided cable). I have known this cable to corrode through. If it has, the easiest remedy, is to replace the entire starter motor.
either clean and tighten cable connections or get battery tested. if not the battery, it is the solenoid or the starter If it is clicking and not starting at first, but starts after a few tries, its your starter taking a dump. Replace it.