Won’t start up

If this is a MacBook Pro (13-inch, Mid 2012) and it is the model with an optical drive (for inserting CDs/DVDs) then you most likely have a failed HDD cable (the cable that connects the hard drive to the logic board). It’s very common for those cables to fail on that model, and the primary symptom you’ll get when they do fail is the flashing folder with a question mark on startup. I would recommend following this procedure to replace the cable. It may be a good idea to cover the exposed areas of the cable after installation with electrical tape to prevent it from future damage (it tends to rub up against the bottom plate and degrade over time). I hope this helps! For additional confirmation that you have a failing cable, you can remove the internal drive, connect it to a separate machine or the same machine using an external USB enclosure, and then use the option key on startup to select it from the startup manager. If you’re able to boot into it connected externally but not internally, you can be pretty confident you have a failed HDD cable.