i had the same problem when my car was impounded a few years back and i was removing it from the towing storage
someone at the tow yard and fiddled with my engine, and driven it (i wrote the kms down when it was towed away) they had driven it till empty, or nearly empty and the gas simply evapourated in the tank while it was sitting.
when i went to pick it up, it wouldnt start, dead battery, put a jumper on it, would crank crank crank, but nothing, got some gas, tipped it in, crank crank crank, waited for ages, kept trying, but nothing
ended up towing back to my house
i took the spark plugs out, cranked the engine a few times without the plugs in (its good to do this but make sure your coil packs are out of the way)
while they are out check the spark plugs are sparking and earthing to the car.
i put it back together and turned the key
vrooom
started first pop
taking the plugs out can be a hassle with subarus but if you take your time its easy as
on the left bank remove all the air intake so you have room
on the right bank remove the battery and winder wiper tank and slightly jack the engine up on the right side (not too much just a bit) will make life alot easier
hope this helps