Engine wont turn over.

Everything is as it has been since I have owned the car,about four years. I did a road trip, 1,800 miles then didn't drive for a day and a half. So I'm not sure how there could be water in the heads. I suspect that it's my solenoid, or some electrical glitch, but I can't find it under the car. Like I said it did the same thing a week before, got it started, then ran fine for several days. Now turn key no sound from starter but there is a humming sound for maybe 15-20 seconds. OK as i thought it was the solenoid, which can be bypassed with a screwdriver touch connecting the two large bolts on it with the ignition switch on. If you have this problem try this before you have your compression tested! Or at least you can drive to the auto parts store to buy a new one.