time machine says preparing for about 5-10 minutes then it says looking for backup disk. Does this mean my backup disk is broken or has a bad connection? the light is on. Its a WD My Passport essential

time machine says preparing for about 5-10 minutes then it says looking for back up disk for ages. Its probably been doing that now for 25 minutes and its done that for a lot longer when I tried before. It used to work. I have a WD My Passport Essential back up drive, about 4 years old. Do you think that's broken? I don't really understand time machine. I just assume its doing what it says it does an its always said before that its backed up.