MacBook Pro (15-inch, Early 2011)

Hello Kay,
I understand that you are seeing a folder with a question mark on your screen when you startup. That is going to be because it can't find your startup disk. Let's see of we can get it sorted out.
For starters, hold the Option key while powering on your MacBook Pro. This will bring up the Startup Manager so you can see if your hard drive shows up. If you see Macintosh HD or if you named it something else, select that option to boot to your hard drive. If you are still having issues seeing it or even booting to it, launch internet recovery to see if you can reinstall OS X. For more details, take a look at the quoted section below for more details.
About the screens you see when your Mac starts up
https://support.apple.com/en-us/HT204156

Folder with a question mark

If you see a folder with a question mark appear instead of the Apple logo, it means your Mac couldn't find a local or network-based startup disk. This can happen if the disk selected in the Startup Disk pane of System Preferences isn't available. Wait a few seconds to see if your Mac is able to locate the startup disk you specified.

If you still see a question mark after waiting a few moments, use Startup Manager to start up your Mac, then re-select your startup disk from System Preferences.

If a question mark appears after you install a software update, re-select your startup disk using OS X Recovery.

Let me know if this helps or need further assistance.
Take care

Hi I'm having a similar problem with mine - however I've tried to load startup manager to no avail, it just takes me to internet recovery. Have tried this with a new SSD drive in the hhd slot and with the HDD in an external caddy as i'd like to know both if my HDD is kaput, and why my SSD will let me format and access it in Disk Utility but not install OSX using internet recovery. Seems there's a lot of people having such problems 😟