1. Where there's a slump, there's a-pus. Where there's a slump, there's pus, icky but effective. I.e., there's usually a reason for getting down, blue, for under performing at work or at school or sexually etc. And if you can get at that reason, at that pus, that infestation, you can know what to target it and target it hard and recover all the more quickly.
2. Let's look first at what we'll have too look at sooner or later. It's possible that your slump is related to mental disorder, permanent or otherwise, and it's maybe a good idea to at least talk to a counselor, describe your situation, get another opinion, and proceed from there. It's not like you're going to be forced onto meds or anything, one of the main positives that you've got a sympathetic, trained friend to listen to. Assuming, of course, that they're sympathetic and/or trained. You don't want to go in there and be told you're in a slump because you were adopted by a family of gorillas as a child, repressed the memory, and are homesick for the jungle. You think I'm exaggerating, and I am, but only very slightly. Still, though, if the slump lasts long enough, and especially if it's a cyclical slump, that is, it comes and goes with some regularity, talking to a good therapist or psychiatrist is something to try. Just remember that they're not God, they're not MDs (well, the psychiatrist is, but that part of his training usually deals with people who believe Vlad the Impaler has possessed their bodies).
3. There are also milder forms of mental difficulties such as seasonal mood disorder, which happens to a lot of people, e.g. they get unaccountably depressed in the winter and never associate it with the fact that their world has suddenly become a cold, grey, wet, snot-infested pit of human suffering.
4. OK, moving on to slumps that could be related to more ordinary things, again that's going to be one of the keys to getting out of your slump. Think hard. Has a cherished relationship recently started to slide? Have you started an addiction which you thought was harmless enough but turned out to be really controlling? How about your age? Approaching a mid-life crisis? Etc. These may seem like roll your eyes, how patently obvious examples, but it doesn't take much to start a slump. I've known people who've gone into a weeks-long slump because they overheard someone talking and misinterpreted them. But they were able to work their way back to that moment, and things improved dramatically. Remember, we're not talking about primate families of kidnappers. Most likely, the cause of your slump is some pretty ordinary (humanly speaking) thing, and you need to address it and move on.
5. But let's say your slump is simply inexplicable. Everything's going great, you've got a good job, a good spouse, good kids, whatever, you've never had a history of mental illness and neither has either side of your family for 6,000 generations, except for one guy, Cain, and he was just weird to begin with. In this case, you're only help (I mean real help, not bad help like boozing it up) is going to be doing things that make you feel physically and mentally better because of the nature of the activity. Jog, hike, play chess, build something with your kids, take your spouse somewhere special, get a sweet new hairpiece, or, in the case of you bald ladies, a sweet new wig, get your whiten your teeth, join a gym and get a six-pack, join a charity, start a charity, work in a soup kitchen, get involved in politics, try ultimate fighting, do something, in other words, that's proactive, healthy, and importantly something that brings joy to your loved ones.