How to attach the drain assembly, in a backwash

my shampoo bowl in my hair salon, is called a 'backwash unit', you stand behind it to shampoo hair...well, the metal drain assembly is 'loose' and water is leaking around the edges. I want to fix it without calling the plumber! The metal drain assembly is loose enough that i can actually pull it out of the basin. It appears to need to be 'screwed' into the basin fixture, but it won't fit tightly. I used this shampoo bowl without trouble until recently. It just started leaking suddenly..I don't know what to do!