I was looking through a catalog for a grow & hydro store nearby. If I remember right the fixtures and the original bulbs/tubes & replacement bulbs/tubes have a two year warranty.
What I noticed was that the LED tubes from ArgoMax were listed as working in any T5 unit. The listing in the catalog says to "upgrade and retrofit any 4' T5 with the new LED line". I did not check the 2' section to see what was written there.
Jonny, take it back to the store and hand it over and tell them that you just got it a little over a year ago. They might fix it even if your warranty was only a year. Does not hurt to try. If you bought it on-line I have no idea of what you can do except try to contact the seller and/or the manufacturer of the light. You might have to fix it yourself or just buy a new one.