Knees and brains usually.

Too many pro-sports leagues use a pay structure that leaves players earning far too little, and the players end their careers with injuries and no compensation.

Depends on the person.

Even if you didn't get injured, or sexually assaulted as part of "hazing", or pick up a drug habit, playing a sport at a professional level will often ruin you.

During high school you spend all of your non-school time training, so no after school job and your grades probably aren't great.
Then you get to university, where you play the sport without being paid, and you one again can't really have a job or focus on your studies.
Then you get drafted (or just hired for some league) into the sport, and you play until your body isn't good enough anymore. You are then discarded with no job experience, no training, and possibly moving teams/cities every couple years so you don't really have any non-sport contacts.

If you get into a league that pays insanely well, you are likely fiscally fine. That's like the NHL, or the NFL. If you only get into the CFL, the WHL, the "minors", or you play in a women's league or a sport that not enough people care about, you probably were not paid enough to survive once the game is over.

So, what do you do with no job experience, no contacts, shrinking savings? You get a job that you can leverage your celebrity, like real estate or car sales, or something that you can also do if you have a criminal record or no CV, like roofing or landscaping.

Edit: not that there is anything wrong with any of those jobs, but starting to learn a new job at 40 isn't fun. Also, if your body isn't in top-shape anymore, roofing and landscaping is brutal.