We know enough from the many victims of strokes to know in broad terms what causes loss of learned behaviour, of even the most basic functions like walking, and talking. The brain has been damaged, and how it works is both complicated, yet very simple at the same time, but victims of brain injury, only need to understand the very simple part, which is that when we learn, we initially burn new pathways between already existing nodes, called neurons.

The pathways start out as tiny almost impassable trails hacked through thick jungle, and turning such trails into massive highways of creativity takes a huge amount of effort. A lifetime of effort invested in the old pathways, and has now been lost through injury.

It's why organisations like the army, do training by constant repetition, until their desired behaviour becomes instinctive "Muscle Memory".
Massive multi lane roadways you have built up since birth, have been destroyed by the injury, but your brain is resilient, and can make new trails between the old nodes, or even other adjacent nodes.

It's simple to understand, and relearning any old skill that you've honed to a fine edge since childhood is possible, by going back to basics.
It's just about the most difficult thing, any adult can do though, especially with creative abilities, where unlike walking, and talking, you may not be aware of all the individual components that go to make up the final result.

The only real way to do it right, is assume you lost everything. Start learning the subject (in this case music) as if you don't know anything about it. Parts you may still instinctively know, without ever thinking about it, (I'm not musical, but for instance, can you still recognise the musical scale, are you now tone deaf, whatever is fundamental to the subject). You can quickly run through any elements you still know, until you come to a roadblock, where something you know you once knew well, has now gone.

It's very hard to rewire an adult brain, and some people will never manage it, because they can also learn new undesirable easier trails, and reinforce those undesirable pathways instead. Stroke victims that never even try to relearn the basics, burn new, "I can't do it" pathways, instead of the "I will do it" ones they should.

You're trying to relearn a lifetime of experience, within a few months, it's extremely tough but not impossible, it just takes an immense amount of effort, and no guarantee of the desired result, which to some extent depends on how much, and what parts of the brain were lost.
The only thing that is certain though, is without making the required effort, you will never relearn any lost functionality, with effort you might.