Without mentioning the year, model number, or any specs about the MacBook Pro, it will be hard to get an accurate answer. On certain MacBook Pros, sometimes you’ll need to reset the SMC, and PRAM or NVRAM to get rid of the culprit. Just search "SMC reset" first, then "PRAM reset" and add the year or model of your Mac.
Here is an article from Apple about
SMC Reset:
Resetting the system management controller (SMC) can resolve certain issues related to power, battery, and other features.
The SMC is responsible for managing behaviors related to the following features. If you're experiencing issues with any of these, you might need to reset the SMC.
- Power, including the power button and power to the USB ports
- Battery and charging
- Fans and other thermal-management features
- Indicators or sensors such as status indicator lights (sleep status, battery charging status, and others), the sudden motion sensor, the ambient light sensor, and keyboard backlighting
- Behavior when opening and closing the lid of a notebook computer
If resetting both didn't fix the problem, and your internal hard drive is still not recognized, I would open up the Mac and replace the HDD, it shouldn't be a difficult task depending on the year/model of your MacBook Pro. There are plenty of videos on YouTube that shows the whole process. I replaced a while back my internal HDD to SSD on my old MacBook Pro Late 2012, it took me about 30 mins total from start to finish. Good luck