If it's non sticky, put some sticky on and replace it. I'm being facetious Gordon
However. without being able to see the problem, then gluing it back on is the most obvious answer. I have done that some years ago with a Minolta lens without a problem. Again it depends on the lens and how it was designed. Is it a continuous ring without a joint? Is it a partial ring with gaps? or joints? Is it flat? narrow? wide? or shaped?
A picture of the lens would really help.
Sony A7, A850, A77, A700 || Sony>> 70-300G, 28-75 f2.8 SAM, 16-80CZ, 50 f1.4, FE 28-70 OSS, FE 24-70CZ f4 OSS, LA-EA4 || Minolta>> 300 f4 G HS, 200 f2.8 G HS, 100 f2.8 (D) Macro, 50 f1.7, 28 f2.8, 28-135 f4-4.5, 70-210 f4, 500 f8 Reflex, TC x 1.4 HS, TC x 2 HS. || Sigma 21-35 f3.5-4.2. || Tamron SP 24-135.