Selenium cells don't usually die. The contact between cell & frame typically go through corrosion, tarnish or even shock. Sometimes the connections are soldered to the cell. I've never been sucessful in resoldering them.
Sometimes a CDS meter can be brought back to life by reheating all the connections with a soldering iron, it may be the failure is just a defective joint.
I've got an old Selenium GE meter from the early days where their warranty states that the technology of the cell itself is so new they don't know how long they will last.