My MacBook Pro (2014 model) trackpad and keyboard suddenly became unresponsive few days ago and I took it to the apple repair shop. Today they informed me it's a top case problem. I am curious, why would a top case cause problem in the trackpad and keyboard. Also, I have been taking great care of the MacBook, why is there a problem in the top case ? Is it because it is a defected unit and can I get a replacement?
While I agree with Tetsujin that you need to ask the repair shop for details, it probably helps to understand some terminology.
"Top Case", at least to me, is a bit of a misnomer. You would think it would be the "top" or the case around the screen not the part that has the keyboard assembly...but hey...it is what it is.
Anyway... the top case is the general term for the entire assembly that has the top cover of the Logic board, the keyboard, track pad, and power button.
When there is an issue with any one of these components, people generally say "it's the topcase" because it is royal PITA to change the individual components out. However, on the MBP 2014 model, you can replace the trackpad w/o disassembling the topcase. It still is a PITA, though.
Here are some instruction on that repair for your reference.
Now..... "why did it go bad?" Any number of reasons from liquid damage, hitting it too hard, or it just wore out.
Is it defective and can you get it replaced? Yes.
The follow up question is "is your MBP under warranty or AppleCare?" If not, then you will be paying for the repair out of pocket.
I hope that sheds some light on the problem.