Uninstalling Sweet Home 3D with the installation program doesn't actually delete all the files linked to Sweet Home 3D: it doesn't delete all the SH3D files you created and the 3D models / textures / plug-ins you imported.
Maybe you switched to another user on your PC or you have a cleanup program on your PC which does too much cleaning.
By the way, the recent projects list just lists you recently opened. If the list is empty, it means that the preferences where this list is stored were deleted, not the SH3D files they point to. Try searching on your PC the files with a .sh3D extension.