My Passport Ultra (bought in 2014) is UNREADABLE by my Mac, suddenly

the WD My Passport Ultra WX71AC3N0499 (wired, product of Malaysia, bought 2014 in Singapore) cannot be read - is unreadable - by my Mac:
I can unlock it by my password, but neither can I open the device-content, nor can I delete (clean) it to install it new.
At minimum, I would want to delete it completely and install all content fresh and new.
How can I make it readable?

WD Passport Ultra on my Mac functions again, just by itself.