Time Machine and external disks

My Mac Mini is running Mojave 10.14.4. Time Machine suddenly says that neither of my back up disks, one of which is always connected, and the other which I connect and separately back up once a month, are available, and they cannot be found. I no longer see the external disk icon on my desktop (upper right hand corner) either. Why won't my Mac recognize the external hard drives? WD Passport and "Porsche Design." Been using them successfully for a couple of years. How can I fix this? Thanks.