The other thing to keep in mind is alcohol tolerance. Adding fresh yeast to a drink that is 14+% ABV still won't carbonate it- even a higher tolerance for the new yeast strain wouldn't necessarily work because yeast added to a high ABV drink will die of alcohol poisoning before replicating and being able to carbonate your mead.
It depends on the OG and the FG and the current ABV, but I probably wouldn't try to carbonate it as most likely the reason it stopped was because it had reached the "finish point" of the yeast strain.
Broken Leg Brewery
Giving beer a leg to stand on since 2006