I'm 11 years late but I was having the same issue (I accidentally hid my main scroll bars when I was playing with splitter bars, thinking I was only modifying splitter bars that I had added).
Fixed it by File>>VI properties>>Window Appearance>>Default
Or, if you have other custom Window Appearance options set that you don't want to lose, you can go into "Customize" and select "show vertical scroll bar" and "show horizontal scroll bar."
This is in LV2018.
