What else could it be!?

Like Mayer asked, "Did the SSD ever work when you placed it in the laptop?" Secondly before you shut the machine down were you able to clone the old hard drive to the SSD using a software like carbon copy cloner? The software is free however the use may be limited. I have done multiple configurations on both of my mac an iMac 2009 and a MacBook Pro (RAID-0 with backup by Time Machine daily, RAID-1 this all with a OWC data doubler and two sats 3 SSDs of 240 GB each.) I take it from your post and I am insinuating this that you failed to clone the previous drive. One idea with further reading of your post try to create a usb install drive with the Disk utilities and everything to install OS X on your SSD. There are instructs on the web mind you Google is your friend. And the whole process should only take you once you find the instructions and figure things out about 30minutes to an hour depending on the size of your Time Machine backup (a fresh install is better on those era MBPs). Also good luck on your repair I believe you'll do well and if need be and you have trouble I have done this before feel free to contact me and I can do a FaceTime session to assist you in diagnosis and troubleshooting the issue.

MacBook Pros, particularly the 13" model in my experience (although that may have been just because they were way more popular...) absolutely have an unacceptably high HDD cable failure rate. However, that's not always the cause of the "flashing folder" issue, which means that the machine couldn't find an operating system to boot from.

If it were a HDD, I'd immediately ask if you'd dropped or bumped the machine. However, SSDs are much more shock tolerant. That doesn't mean you can throw one across the room -- solder joints can still crack very easily -- but it does make it less of an issue. Still, if you've dropped the machine, that may clue us in to what's going on.

SSDs have had a rough start -- they're fairly new tech -- and certain models have had high failure rates.

What model of SSD do you have?

Also, if your machine has this capability (it may be a year or two too old), check to see if your drive is being recognized at all. You can do this by holding CMD + R right after powering on the machine. If it does happen to boot into the recovery OS, use Disk Utility (listed under "Utilities" on the menu bar, or it may be one of the four options immediately showing on your screen in the OS X Recovery environment). If your drive appears on the left, that likely means the OS just needs to be reinstalled. If it's not showing, you're more than likely going to need a new drive.

Rarely, the issue can be caused by a logic board-level component failure ($$$), but that's something like 5% or less of the time, in my experience.

This usually indicates that the OS cannot be found which is (in most cases) caused by your HDD failing. You need to replace the HDD, hopefully you have a backup, then restore from your backup or if you aren't too fussed about what's on there, do a disk utility partition reset then reinstall via the apple store over wifi.

Start it up normally then hold the Option key for a bit. Eventually a menu will appear and see if you can select your normal startup drive in the list. If it boots properly, then you can go into the settings and configure your startup drive. Might have just messed up somehow.

If it doesn't detect or doesn't boot still, you might need to repair the drive.

Follow this for a legit tutorial: Apple Official website

Mostly need to check the following:

1. If your Mac doesn't get past the flashing question mark
2. If your Mac starts up to a gray screen (with no flashing question mark) or a flashing question mark or flashing globe, give the computer a few more moments to locate its system software. If you normally start your computer from an external drive (Thunderbolt, FireWire or USB) make sure the drive is connected and powered on.
3. If your computer still does not start up, try the next set of steps to help your computer find its system software.

''Turn off your Mac by pressing and holding its power button for a few seconds.

Press the power button once to turn your Mac back on. Then, hold down the Option key on your keyboard until the Startup Manager appears.

Select your startup disk from the list of drives that appear on screen.

If your computer finishes starting up normally, choose System Preferences from the Apple menu. Then, click the Startup Disk icon in the System Preferences window.

Select your normal startup volume (such as Macintosh HD) from the list of drives that appear in the Startup Disk window.''

If your installation of OS X needs repair

If you don't see your startup disk displayed in Startup Manager, or if you see a prohibitory symbol (⊘) appear, your startup drive may need directory repair, or OS X may need to be reinstalled.

''If you only see a disk named "Recovery HD" in Startup Manager, select it instead. You can also start from OS X Recovery by holding down Command and R on the keyboard at startup.

From the Utilities menu, choose Disk Utility.

In the Disk Utility window that appears, select your startup disk (usually named "Macintosh HD") from the left side of the Disk Utility window.

Click the First Aid tab.

Click Repair Disk to verify and repair any issues with your OS X startup disk.

After repairing the disk, try to start up normally.

If none of these steps resolve the issue, start from OS X Recovery and reinstall OS X.''