I think the situation usually comes about when someone underestimates something, or something unexpected throws you off course. Then all of a sudden, you've got a ton of deliverables and no time. It's easy to provide good service if things are great, but if your boss suddenly demands that you put in a ton of extra time, then what happens to the side projects?
At that point, you either front up and say something like "Look, I'm really sorry but I'm in a big bind, {whatever} happened and I'm really behind and I'm gonna need some leeway. Here's my plan to get everything back on course with a {blah} delay. If that's not acceptable, then {some other option}." And you have to be able to accept that they might give you a really hard time.
I'd like to think, if your problem is genuine, most people will respond positively. That's gotta happen quickly though, if you start jerking around, you're not gonna get sympathy.
Not communicating though, is pretty much the worst thing you could do. That's going to build up serious aggro, suddenly things like lawyer letters and fraud charges come into the picture very quickly.