Ok well this is shit you definitely need to know so listen up, dawg.
In your body lives a little dude called your HPTA. It stands for Henry Parsons Turlington Adams. He's a great little guy, and regulates all kinds of cool shit in your body. When he's all switched on everything's groovy, particularly with regard to your hormones.
Steroids are endogenous hormones, and when you introduce exogenous hormones into your system, Henry senses this. Henry knows that if you're supplying hormones then he's not needed, so he goes for a nap.
The problem is he's a bitch to wake up, and he will usually wake up on his own once you stop the hormones, but it can take a really long time. SERMs like clomid and nolva can give him a good shake and wake his lazy ass up sooner, but it's by no means certain and sometimes our buddy Mr. Adams never wakes up properly again.