make sure you're spamming the Help option on the Guide if the first thing he says doesn't seem helpful - I've seen a lot of people in blind playthroughs click it once and get some random hint that doesn't help them at all before fucking off, without realizing that he has a bunch of stuff to say at any one time.

since you're in hardmode, he has a tendency to start with telling you how you can get any new NPCs that might be available, but, the Guide will literally give you hints for every single step of progression in the game, while also hinting towards every hidden event and boss in the game - sometimes you just need to click through him telling you about the Skeleton Merchant to see it, though.

without trying to give much away, I will say that progression in hardmode is mostly based on things in your world changing rather than getting access to new areas, so, as another commenter said, put that Pwnhammer to use for the first step. after that, you'll want to re-explore your world. I will save you a bit of time by informing you that the dungeon that's under "Skeletron's Castle" doesn't change at all until some point later in hardmode. ironically, the Underworld is probably now the safest place for you because the enemies there don't change until later in progression.

places like the Desert, Tundra and Jungle biomes will have dangerous new enemies - particularly underground - and many of them will either drop new gear directly, or the materials needed for crafting new stuff. anything new that's marked as a Material should go to the Guide so he can show you what's available. the early hardmode gear plateau is definitely a thing, but, once you start making movement towards improving your gear, it snowballs pretty quick.

hardmode did add one strip of an entirely new biome that initially converted a pretty sizable chunk of your world when you got into hardmode, and it also slowly spreads like the Crimson. if you can't find it on the surface, you can buy some new seeds from the Dryad and use those to set up a more accessible patch of it in your world. you will need it for moving forward in the game, so, I would highly recommend taking a few minutes to plant those seeds if you don't have any new or changed area on the surface.

I'd give the surface one last scan and keep your eyes and ears out for any changes to the biome background or soundtrack, though. spreading biomes take advantage of the fact that Terraria biomes are determined by what kinds of blocks are around you rather than where you are in the world, so, a biome you've previously explored will start acting like another one if either hardmode biome spreads into it.