Great question, and very relevant for a lot of people! Cell damage can occur in lots of ways, and the type of damage often will dictate "what happens next."Let's say that there is some kind of trauma to the muscle, such as a cut or crush. If cells actually break open, this is something called necrosis, and it leads to an immune response - the body's surveillance system detects the cell"parts" and essentially cleans up the mess. This is part of a wound healing response. There will be generation of new muscle cells, but if the wound is large or the person is compromise in other ways, there can be scarring. There are other ways that cells can become damaged, and this has to do with nutrient and oxygen availability. In mammals, including humans, cells harvest energy from glucose, and they can do it under aerobic (with oxygen) or anaerobic (no or little oxygen) conditions. Skeletal muscle, made up of fast and slow twitch muscle fibers can function under anaerobic conditions y using lactic acid fermentation, but this can become a problem after a while - it lowers the pH of the cell, which eventually shuts down the cell (and is the cause of cramping). Too long without oxygen, and the cells die.

Muscle cells, like most cells in the body, can repair some kinds of mild, internal damage.It depends on the type of damage and how severe it is. Things like small holes in the cell wall, or a single broken DNA strand, can often be repaired. If the damage is too severe, a process called apoptosis kicks in and destroys the cell from inside before it can harm its neighbors. Apoptosis is why you shed some outer skin after a sun burn.

One unusual thing about muscle cells is that if you work the muscles very hard, like lifting weights, you actually cause microscopic tears *between* cells--although not inside a cell. The body responds by adding more muscle cells, so the muscle gets bigger and stronger. But large tears like a "pulled muscle" are bad. They can trigger apoptosis (and other effects) in the damaged muscle cells, which will make the muscle weaker and leave scar tissue behind.

Skeletal muscle cells are like nerve cells (neurons): once they are made during the initial stages of development, they never reproduce again. So you have a fixed number of muscle cells in your body, and if one dies, it is not replaced. However, individual muscle cells can grow and shrink by increasing or decreasing the number of muscle fibers they contain. So if a muscle cell dies, its neighbors could expand to fill the space (and take on the workload). This expansion and contraction is also the way we build (or lose) muscles. Bodybuilders don't have any more muscle cells than you or I do, but their individual muscle cells contain many more fibers per cell.

I should also mention that if a muscle cell is damaged but not killed, it may be able to repair itself by creating new proteins (or other cell component parts) to replace the ones that were damaged.

Thanks for asking a question!