When I clicked Shift ~ , navigation worked at first, but when trying to scroll to increase speed I accidentally clicked the middle mouse, it made the camera jump into the mesh and now WASD and Arrow keys no longer work (even after undoing) and I cannot move my camera anymore, just rotate it in place. How do I reset this mode to work like earlier?
If you find yourself inside an Object, you can use your Scrollwheel to 'back out' again, or go to View > Navigation > Walk Navigation to get the WASD keys working again:
Maybe even using the SHIFT~ again would return to Walk Navigation, can't test that because ~ doesn't work on my keyboard.