When someone or something prevents or interrupts a wild session of jacking off, in any of three ways:
1) Something preventing jacking off from occurring, i.e. your dad won't get off the computer or go to bed, so you can't get on and look at porn.
2) You have a mental block and have trouble imagining sex in your mind.
3) If you are already jacking off, and someone walks in at the worst moment, just as you are about to cum. This can either end with an abrupt ejaculation which is not pleasurable and hard to conceal, or the orgasm stops completely. The second way can lead to a nasty case of blue balls.
A: Dude I was jacking off last night and my mom scared the jizz out of me when she walked in!"
B: That sucks dude. Total jacking block.