Snow Leopard to El Capitan Failure

Do you have access to another computer running a newer OS where you can sign into the App Store using your Apple ID. Then try the download load to see if you can download El Capitan and make a bootable USB?

See if this download link gives you a working installer.

El Capitan

If it get stuck again, try shutting down the computer using the power button and then restart. Sometimes that will activate the installation again.

Do a backup before installing, preferable 2 backups on 2 different drives. Download the El Capitan installer and then quit it. You can make a bootable USB stick to install using these free programs which will do all the work for you. Boot the computer with the option/alt key held down and select the USB drive. Run Disk Utility and select First Aid. Erase/reformat the drive as Mac OS Extended 9Journaled) with option GUID. Then install the OS.

Bootable USB Flash Drive – Diskmaker X      Disk Creator   or use Apple’s method       Create a bootable installer for OS

Make a Bootable OS X 10.8 Mountain Lion USB Install Drive

Based on what I have seen of updates on this web site vs clean install - you may be better off doing a new install to your reformatted disk.

If you want to keep Snow Leopard - you may want to partition your drive.

You aren't doing anything wrong my friend, however something is clearly going on.

Did you replaced your original hard drive with a Solid State Disk (SSD) or a regular Hard Disk Drive (HDD)? I'm guessing you probably replaced it with an SSD and any off the shelf SSD these days is going to be a 6G/s SATA III SSD and unfortunately that can cause problems with 2009 and 2010 MacBooks like what you are describing.

Assuming you did get a new SSD, you are supposed to use an older 3G/s SATA 2.0 SSD for a 2009 MacBook internal drive replacement which is definitely not something that is at all obvious.

What I'd like you to do to possibly confirm this is the issue is trying booting up your MacBook Pro holding the COMMAND-V keys at power on for "Verbose" mode. This will show you a bunch of text as the OS X tries to load and if you can take a screenshot of the last screen and post it here, that should confirm what's going on as then I can see where it's failing to continue loading.

Try downloading it in Safe Mode (hold down shift while booting) or by using another administrator account.

Are you using Wi-Fi to download this? If so, try a hardwire Ethernet connection (Either CAT5 or CAT6) and see if you get a full install.