Vertically swimming fish?

Why is my Fancy guppy swimming vertically

Doy you mean it swims from the bottom of the tank to the top and back down again or that it always has its head pointed up (or down) whether it is swimming around or not? The former sounds like just odd behavior, the latter, like a bad swim bladder.

The fish swims with its head up all the time! I believe this started shortly after one of my other guppies died!

That sounds like his swim bladder no longer adjusts properly. The swim bladder servers the same purpose as the ballast tanks in a submarine. By squeezing the air in it smaller, the fish can make itself less buoyant so it's heavier than water and sinks or can relax, let the air expand and the bladder get bigger so the fish floats up.

It sounds like your guppy may have lost that control and his is always too big and so the front end of him, where the bladder is, floats up. Some folks have said they have seen those sorts of disorders heal themselves. I have never seen that myself and found those conditions to be permanent. I know of no treatments for it, sorry. The good news is that some fish can live a long time (sometimes their whole lives) with that handicap as long as they can get around and eat.