Double check the external hard drive to make sure it is still connected, and mounted.
On Windows, the most troubling problem with external drives is that drive letters tend to change constantly. And Lightroom expect those to stay constant (sic). But if you work on a Laptop AND a Desktop, having a mobile disk is the best solution to go.
I assign my external hard disk a drive letter that is high enough, so that Windows does not allocate this one to a USB-memory stick or some other external drive that got plugged in before. Very "bad" are those "11 in one" card readers. They tend to allocate a bunch of drive letters at once.
There is also a possibility to attach a drive to a path in your current directory tree like:
...\name\My Documents\My Pictures\Photos.
in this case Photos needs to be an empty directory. The disk works then as an extension for that path. Instead of Y:\... you access your data at Photos\...
More may be found here: Assign a mount point folder path to a drive
Than you for your help and thoughts re drive letters. This is exactly what had occurred and all is good now.