i did what @jimmymcgoochie said and it worked! all parts are back, thanks! it took a lot of time to reinstall everything
i didnt knew i was supposed to isolate KSP from steam, thank you for the help.
apparently, USI is not supported in the current version, probably what caused steam to delete everything. i had to uninstall it.
again, sorry for posting in the wrong forum and thanks everyone for the help!