Just end the dive.If the computer shuts down, what do they do? Perfect recipe for panic if you ask me.
I usually have an idea what my NDL will be prior to jumping in, so if it fails I should know where I was NDL wise at that time. In any case I would just start heading on up.
My main concern is the amount of new divers that I come across that has no computer and have seemingly no care or real knowledge about NDL. They jump in and end the dive when they run low on air. Oblivious of what their NDL is. Many times I watch them deep below me as I slowly ascend during the end of the dive as per my computer NDL. When I suggest that they need to start planning and executing dives with NDL in mind, maybe even buying a computer.... blank glossed over eyes are all that I see.