I've killed the Wall of Flesh but can't get to the loot

Solution 1:

While the Pwnhammer is instrumental in getting easy access to hardmode ores, there is another way!

Crates can be fished from the water! These crates have a chance to contain hardmore ores, allowing you access to those more powerful armor and weapon pieces. Depending on if you're playing the PC, Console, or Mobile versions, your access to certain crates may be denied, as the Console and Mobile versions only have Wood, Iron, and Golden crates.

Better bait and better fishing rods will give you greater chances at obtaining crates from your fishing excursions. The Angler's quests will give you access to better rods and bait over time, but these can take some time to complete. Once you're better geared up, you can make your way down to the Underworld to get your WoF loot, and maybe even fight it a few more times.

Information gained from here.

Solution 2:

The traditional way to reach the Underworld level, post-Wall Of Flesh, is to use a Hellevator: basically a straight-vertical two-block hole in the ground, leading all the way from your home base down to the Underworld. You can buy a bunch of rope from the Merchant to let you go up and down easily. This lets you avoid monsters because (1) you're moving past them very quickly, and (2) most of them can't get at you through walls.

Usually you build your Hellevator before starting Hardmode, but you can probably get it to work afterward if you're careful. Make sure to line the sides of your Hellevator with blocks -- you don't want monsters wandering in! There are various ways to deal with pools of water and lava in your path, but my favorite is to drop bombs until it breaks through and drains the water/lava into a cavern somewhere below.

Burrowing and phasing enemies will be a problem for you, especially if you get too close to the Hallow or the Evil biomes. All I can suggest is to Magic Mirror out and come back when they despawn.

Once you get to the Underworld, things will get easier -- no Hardmode enemies spawn naturally in the Underworld at the start of Hardmode.