Most common belief is that your muscles might turn to fat. That is absolutely not how it works. While you work out, especially to gain lean muscle tissue, you need to feed yourself in order to do so.
While you consume more calories, you also burn more calories from your workouts. The problems come when you stop training but keep eating the same number of calories. It doesn’t happen overnight nor within the next week but still, it is the most perfect way to gain fat. So, your muscles aren’t actually turning into fat, you are just eating your way into fatness.
The muscle though, will lose something. Glycogen and water are stored in muscles and after only two weeks of detraining, like no strength training whatsoever, muscle glycogen can drop by as much as 20%. Imagine your muscle being a grape when you train regularly and a dry raisin when you stop training.
You’ll also lose some strength, which is obviously normal, but every muscle has a memory. If you stop for a short while, as soon as you step back in the gym, they will grow back faster than it took you to gain that strength the first time. Just don’t go at it sporadically because you won’t get gains like the regular and more serious gym goer.