Not being able to get stuff done immediately is only human. OTOH, not communicating immediately with people who are waiting for you to do something is much less excusable. Virtue, courtesy and kindndess aside, just from a practical point of view, it annoys people. Even a message, "I'm having some difficulties right now, but I haven't forgotten you" helps a whole lot. Nothing infuriates people like the feeling that they have fallen into a black hole...