MacBook won't boot from internal SSD
I don't know how it worked without issues for over a year... but the culprit is probably the drive cable. To put a long story short, the interface is SATA III on the motherboard, it's SATA III on the SSD, so they will talk SATA III.

But if your MBP shipped with a SATA II HDD, it most likely came with an interconnect cable only rated for SATA II.

I wouldn't have thought this would be that big of an issue (I sort of thought the difference was like the difference between a CAT 5 and CAT 5e UTP) but apparently with the models of SSD's on the market right now and their throughput, it causes an issue. 

I couldn't tell you how to determine if the cable's the "updated" SATA III version or not when shopping for parts, but I do know that this one is.