Help Boot up always goes to BIOS

Background

I briefly moved the system SSD's SATA cable to a different motherboard socket (in the same block) because I was going to clone the drive to a new larger capacity SSD using my True Image 2016 software. I connected this new larger capacity drive into the socket previously occupied by the system SSD. This seemed to be the advice from the Acronis forum. I was going to use the Clone Drive utility. For safety's sake I took an image copy of my system SSD before starting.

However, when it didn't boot I confess I chickened out, and so I returned the system SSD's SATA connection to how it was before, and disconnected the new SSD.

The Problem

Now, the boot-up process always goes to BIOS. If I then quit BIOS without saving, the computer boots into Windows 7 just fine. Obviously, I've screwed something up, but what ?

Help please. I was reasonably au fait with the old style BIOS implementations, but am completely new to UEFI.

Best Regards
Graham

My system
Asus X99-S motherboard
120gb Intel SSD for system/programs
512 and 256gb SSDs for projects and data
Just bought a Samsung 850 Pro (256gb) from Scan onto which I hope at some point to clone my system/programs. Once the above problem is solved I will raise a separate thread for advice on how to complete this.