Originally posted by Vlad Akhmatov:

Well, i lost all my buildings and equipment. I've read the fixes, but they leave out important steps. Would someone please give me detailed instructions on how to recover my stuff? I've put over a thousand hours into this game. That may be it.

Well, for me it was the files in the appdata/locallow/irongate/valheim folder. As mentioned above by Polonius Ulf, there is a shortcut to that folder on the startup screen. It is a bit hard to detect, it is a very small white writing in the left bottom of the main menu.

There you'll find folders like "world" and "character". If you are lucky, there are files in the "world" folder that have the ending ".old" (look at the date of these, I hope they are older than when your loss happened). Delete the ending ".old" from these files names.
What I did was to delete the current world where all my buildings and everything had disappeared, and then I did the described steps. My world was shown in the seed selection again, in an slightly older state.

I hope that works for you.