Unable to boot as drive is inaccessable

When you power up the PC you start a chain of programs finding and loading each other until the last loads Windows 7.
BIOS (the motherboard's firmware) goes and loads boot programs from a drive's MBR, that will then point to other boot programs inside a partition that will finally load the Operating system proper (windows 7).

This compartimentalization allows the hardware to be independent from the operating system.
This is complex but allows many benefits. You can easily install another operating system like Linux if you like, or do a multi-boot machine with 2 or more different operating systems on the same hard drive.
In case malware rapes and nukes your Windows 7 installation you just need to reinstall it, but the hardware cannot be damaged by it thanks to this compatimentalization.
For smartphones/tablets/whatever running Android and iOS it's not like that, there the operating system runs directly the hardware without any compartimentalization. Thus bad/buggy software can damage or destroy the hardware.

In your case one/more of the programs of this chain was on the HDD and not on the SSD, so without it one of the booting programs could not find the drive where the next step would have been.
Glad you fixed it.