If something serious is preventing you from being there for your best friend(like being in a coma or something) then that is understandable. But I think in most cases, it is simply people not caring enough. Being busy with work or this or that is no excuse.
Of course it's an excuse, it's work for crying out loud. Do you have any idea how hard can it be to get a job nowadays? That set aside, who forced you to help or ... Well do pretty much anything for your friend in the past? Did he / she want any of what you did at gunpoint? Did you have freedom of not helping him/her? Yes? Well in that case, you've done so because you wanted to. And now you'd hold that person in some kind of self-imposed debt? This entire idea of any of your friends actually being in any way obliged to help you when you need it sounds rather silly.
If they don't help you, well, that is tough, but it's you who's to blame for putting energy into them in the past without getting to know them better, for believing that they won't turn their back on you the very minute you want to be on the receiving end. Even if you were friends for a long time in the past, even when you were helping each other for years and the help was genuinely mutual, it might just happen that the other person's going trough stage in a life that's very tough or just unbearable and doesn't even tell you about it. If the help was never mutual and you were always giving, well... You should have seen that coming. This entire thing reminds me of a friend who was brutally friendzoned by one girl - I've kept poundig all this into his head and he, thank god, understood eventually. But got incredibly hurt during the process.