Starter Not Cranking

In order of likelihood, bad ground, bad/weak battery, bad starter, bad starter solenoid, seized engine. By the way, I'm assuming the single click you hear is coming from the starter.

I'd check your connections, and if they look ok, you could: Directly connect battery (briefly, only to start) to the starter connections to bypass the ignition and narrow down the problem. Also, you could tap the starter with a hammer and see if it then starts. If it starts, the starter is likely the culprit. Good luck!