Why won't my car engine start?

I'm an automotive rookie, but here goes... 2000 GMC Jimmy, 4WD Was running fine. Changed oil. Still ran fine. Pulled out all my fuses to check them. Replaced a couple of 10amp (red) ones that had blown. Put them all back in. When I went to start it a few hours later it wouldn't start. Engine turns over but does not fire/engage. Thought it was a dead/weak battery. Tried jumping - no luck. Took battery to Autozone. They tested it and said it was good. Charged it, they said it had a full charge when I picked it up. Put battery back in - still no luck. Tried spraying ether into air intake. Didn't help. Checked oil - has a ton, maybe even a little more than it should. Added gas, just in case. Any suggestions? The other oddity is that after a few tries it seems like it gets weaker when turning over. That says battery issue to me, but the battery is supposedly fine.