that happens in 1.4 if you move a product system and do not insert it back again. Making it visible is possible but requires use of the developer tools. Basically in SQL:
select * from tbl_categories where model_type = `PRODUCT_SYSTEM`
-> shows the categories in use for the product systemss
select * from tbl_product_systems
-> shows all your product systems; find the one that is lost
update tbl_product_systems set f_category = ###any the number from the first query that you see as category### where ID = ##here the number of the hidden product system###
-> update to a category that is shown
And I would recommend to switch to a newer version of openLCA!