Unable to start up Macbook pro normally

Ok thanks. Briefly put, when you upgraded your Mac to Catalina it also reformatted the aftermarket SSD to use APFS, a file system format Mountain Lion does not recognize. As far as the ML installer is concerned a startup disk might as well not even be installed—which is what you're seeing.

Assuming the aftermarket SSD, its SATA cable, and the logic board are all operational, what you need to do is boot macOS Recovery as you did and choose Disk Utility. If it recognizes the SSD all is well so far. Then, select the startup device (the upper-leftmost device in the device list, not a volume subservient to it) and format it using Mac OS Extended (Journaled).

Then, you should be able to return to the main OS X Utilities menu and reinstall OS X.

If your Mac starts up to a question mark has seven steps. Follow each one of them in order.

I.K.A wrote:

Hello,

i was working normally on my macbook pro (catalina), then it started to get slower and slower, so i restarted it. When i did so, a folder icon with a question mark screen viewed. I followed the instructions on apple support website, and i made a recovery using disk utility, but that did not solve the problem. Any advice?

You do not say what exact Mac is this ...?

Sounds like the hard drive is failing/failed ...

I.K.A wrote:

It’s Macbook mid 2012

If this is the OEM hard drive that shipped with the Mac when knew— it is a good guess it has failed, typically you can expect 3-5 years on a rotational drive.

This is user upgradeable to an SSD— which in comparison will be lighting fast upgrade .

Advise to swap out the cable at the same time if you move forward with the upgrade:

SATA replacement 13"MBP mid 2012

https://eshop.macsales.com/item/Apple/8211480/%20%2013%22%20MacBook%20Pro%20SATA

OWC SSD Upgrade Kits For MacBook Pro 2012

https://eshop.macsales.com/shop/ssd/owc/macbook-pro/2012

I.K.A wrote:

I already replaced the one came alongside the laptop. I am currently using crucial SSD

So the question would be, did you change the SATA cable(?) It is a known weak point in the equation...

A definitive test would be to pull the SSD and put it into an external enclosure and try booting from there, holding the Option key. This will bypass the internal SATA cable.

It's probably already lost. Did you not turn on Time Machine?

How did you populate the Crucial SSD's contents to begin with?