Legendary's upgraded to Ascended
Short answer: You will need to contact Support (links at the top and bottom of this page) and explain the problem to them to get it fixed. (They can do this and have done it for other people in the past.)
Long answer: Ascended weapons were introduced, and legendaries upgraded to ascended, in the September 2013 update. Before then transmuting a legendary was the only way to get different stats (and the default ones weren’t very good) so almost everyone with a legendary had transmuted it. So, Anet mailed everyone with a legendary a transmutation splitter so they could get their original (and now upgraded) legendary back.
They then, in April 2014, introduced the wardrobe system which completely changed how items are transmuted and made splitters redundant, so they were removed (or rather converted into transmutation charges – the equivalent of transmutation crystals from before the update). When you clicked the transmutation splitter it will have been added to the transmutation charges on your account.
In general this was a good move, but it does create a problem for people like you who had transmuted legendaries before the update and who didn’t log in between September 2013 and April 2014 to convert them and now have no way of doing so.
“Life’s a journey, not a destination.”
Sweet response Danikat, yes I contacted support for this already, I guess ball is in their court. Thanks for the thorough reply.