You lose touch with someone when you no longer have contact with that person. I might say: "I have lots of cousins living in America, but I have lost touch with all of them." If you no longer have any contact with the people you worked with in a previous job, you might say: "I have lost touch with all the people at Smith & Co. except my old boss, who happens to live very near to me." It is normally used of people. For a subject, you might say: "I am now out of touch with modern developments in the nuclear industry, although I worked in that industry for 10 years."