I upgrade my web hosting to a bigger plan and now all my themes, plugins etc etc are gone. My posts from the last 6 hours are gone, certain forums are missing for some members and some members can't even view the home page.

Some members can still see the theme, but no plugin. Others can see plugin but no theme. Most cannot see any of it.

 

I have tried on 4 different computers, 3 mobile devices after clearing my cache/history etc and it still doesn't work. What went wrong?

Sounds like a question that only your host can answer, but I would guess that, if you switched to a new server, that all the files and folders and/or database didn't copy correctly.  

Could be a DNS issue as well, maybe not fully propagated