What has changed?

Have not played for some time.
Only about 200 hours gameplay.
But I remember archer or witch was quite a nice start to get past first few bosses, even the Warden was easy.
But that damn fire throwing second boss kills me at least once or twice with any build I try.
What has changed?
Also why do all the items I see disappear and I cannot pick them up to sell?
I know for most of you who play a lot silly questions.
Still would like them answered.

Who is a firethrowing second boss? If you're talking about the one in the Coast (area right after lioneye watch), there's nothing to worry about. This boss is completely optional with the only reward being a couple of items with a bit higher ilvl.
Perhaps, the change is a bit more damage taken in acts.
There's a button that shows/hides labels of items on the ground ('z' by default).

Press alt to see the items your item filter lets you see.
Z shows everything.

I would suggest you:

Read the basic description of each league here:
https://www.poewiki.net/wiki/League

and/or watch Ziz speed run explanation of league mechanics:
https://www.youtube.com/watch?v=ZosV7m4XkPw

Can't remember which league, but sometime in the past 4 years they did a pass on the campaign making mobs harder. They started with the early acts, I think they might of done 1 and 2, then stopped. So if it's been some time since you played, that may of been one of the changes.

You can just run past that boss.

With the items, you probably need to toggle the key that shows them permanently. It's in the input settings, think it is called 'Toggle highlighting'. Or else holding alt should show them so you can pick them up.

Build's have changed a bit, so your previous leveling builds may not be the strongest levelling builds anymore.

Also, check you're not in Ruthless mode.

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Exorable#0615 wrote:

...
But that damn fire throwing second boss kills me at least once or twice with any build I try.
...

Fire Fury, Boss in the 2nd Area Act 1. Just hug the wall and never move towards the center of the area, to avoid her, or wait until you got your quicksilver flask from the tidal island quest, movement speed on boots also helps (more dps of course too).