I doubt knowledge of explosives would be lost. The advanced ones, and perhaps manufacturing for the more complex ones, would be quickly left behind. But, the knowledge that we had them, and some of the things that could be done with them - from guns and bombs to mining and demolition, there would be too many uses not to try to recreate explosives eventually. Most people would remember that explosives existed, and perhaps a little of what they could be used for, even if they don't remember chemical formula and manufacturing - and such stories would linger for generations and inspire people to experiment, unless some very specific targeting of people with that knowledge was going around.
Additionally, there would be large chunks of the population that do know enough to recreate, at least, gunpowder - including survivalists, reenactors, history buffs, storybook readers (certain kinds of medieval-ish or fantasy has reasonable clues), and the randomly curious. Some of these people would be more suited to survive than others - but some might end up in a pretty good position to spread that knowledge around. Some of those skills - survival skills, ways of hand-crafting any kinds of useful supplies, preserving, are all going to be immediately valuable enough that people will make time to keep knowledge alive, even bits that are a little more useful for the future. People would also be scrounging for useful knowledge - including books that would contain those intermediate skills that will allow people to make the jump from food/water/shelter to rebuilding an advancing civilization. It is possible a given group might not contain any of them, though the high proportion of other useful skills they could offer makes that a little less likely.
As for what would get rediscovered first... gunpowder is a reasonable guess, which can be made by experimenting with mixes of charcoal, sulfur, and saltpeter - all of which are naturally occurring, and two of which anyone knows what it is when they find it and two of which can be created anywhere (knowledge of saltpeter is the limiting ingredient, I think, but it forms naturally in stables). Put it under pressure and set it on fire, and there you go. boom. This is among the most likely of the explosives to be remembered.
Another possible guess might be flour (or sawdust or the like) creating a dust explosion - this would probably be more useful for signaling or scaring people away than causing damage, since it depends on igniting the suspension of the particles in the air. However, it depends on nothing people can't immediately get their hands on, and it only takes one vague-ish recollection that it can be done (or a bakery or mill fire) to get the point across. On the other hand, if someone manages to hang onto a bit more knowledge, then guncotton or nitrocellulose might be a fairly good explosive to keep on hand - it requires cellulose (paper or cotton) and nitric acid - and if you've got saltpeter, you can get nitric acid.
It might take a while for the explosives to actually get reestablished - depending on when they have luxury time to experiment, instead of pure survival. But any group that knows the recipe (or any other handicraft skills) would do their best not to let it get lost, because those skills will be invaluable once the group has managed any stability at all to think about the future. If they can reach medieval level, or even pioneer level, they will probably be experimenting with explosives or crude chemistry. If they're stuck at hunter-gatherer level, it might take longer get around to it.
If you don't want explosives, you might start with a group which didn't have anyone knowledgeable to begin with, or no one remembered what saltpeter was, or set the story in the time before they got around to experimenting yet. Or have someone deliberately working to deny knowledge of explosives, for whatever reason. All of these will slow down the re-discovery of explosives... I just don't think it will stop people from eventually figuring something out. Like I said, there will too many people who remember what useful things can be done with them, to keep people experimenting.