Can no longer boot from external Linux Mint drive.

[Conster]

Okay, not really too sure where to put this one, so apologies if it's not in the right place.
I took the hard drive out of my barely functional Dell laptop and installed Linux Mint 17.3 on it so I could have a full separate Linux Mint installation for my PC using one of those HDD to USB adaptors. It worked absolutely perfectly and I was able to boot in and out of the different drives no issue. However, the next day, I can no longer boot into the Linux Mint drive. It's detected in the BIOS and everything, but when I go to boot from it, the PC just simply boots from the Windows drive. I've tried multiple times to boot into it, but it simply does not. Not too sure what the issue is, the only thing I can think of is the boot loader has somehow gone missing. Any help is appreciated. Cheers.

 

[JMW22]

Can you go into your bios and verify that it is set to boot from USB and that USB devices boot before internal drives? If this is not the issue, the drive could simply have gone bad, or the USB adaptor could be malfunctioning. Also, please verify that the USB port the HDD-USB adaptor is connected to is functioning properly by attaching a USB device to it and making sure it works. If your HDD-USB has a light that indicates it is receiving power this will not necessarily be enough to indicate the USB port (or its associated hub) is functioning.