No replacable fuse in ballasts that I know of .....
You sure it is not the harness/lamp holder? It is more than likely that a wire is loose or the endcap (lamp holder) is corroded. Assuming you have some electrical skills, the simplest way to check is to swap the 2 endcaps around. The endcaps are wired to the ballast inside the lamp using wirenuts. Swap the good one for the bad & see if it lights. If it works, then it is the endcap or the wires. Buy a new set to replace. Otherwise, your ballast is toast.
<Mark all wires so you know how to wire the thing & make sure all unused wires are capped during testing. <And unplug the light when working inside it.> .... If you never work with eletricity before, get someone experienced to help you. Do not try this if in doubt ... It is not worth killing yourself or starting a fire!!!>
BTW - is this the same light that is not working in your other post?
80 gal FW with 30 gal DIY wet/dry/sump.
9 fancy golds, 1 hillstream loaches, 1 rubber-lip pleco (C. thomasi), 3 SAEs, small school of white cloud minnows, planted.