Welcome to the community, @Christine_50!
In addition to what @USAFRet & @R4DIO4CT1VE mentioned regarding checking
Disk Management in Windows on your computer, I'd also advise to try connecting the external HDD to another working PC/laptop and see how it will pop up there. It might also be a good idea to swap the USB cable in case multiple systems have issues recognizing the HDD.
Make sure you backup your data from it as soon as you gain access. This is how you will avoid any potential data loss. Keep at least two duplicates of your files in different storage locations.
Keep us posted with the details & good luck!
SuperSoph_WD