External hard drive not visible/ help

What format was it in, and could a Windows machine possibly read it?

Disk Utility should be able to set it up again, but that would involve erasing all contents. It's pretty concerning if the drive is not readable anymore, as that suggests more serious problems. Drives don't usually get corrupted out of the blue.

 

If it's a Time Machine backup drive and the file allocation table is messed up and the directory needs to be rebuilt, it's best to erase and re-format it. You can't depend on using that drive to restore from after that. Just start over or not use that drive any longer for Time Machine.

 

What filesystem is on the drive? Modern journaled filesystems are very resilient about handling unplanned power failures or disconnects.

PS: you might get better help by moving this to an Apple OSX forum - since this forum is about Apple and industry trends.

 

You might try leaving it unplugged and shutdown the Mac. When you boot back up again, wait for the system to be fully ready, then plug the drive back in.

If that doesn't help, unplug the drive and do a NVRAM reset or Safe Mode boot, then boot normally and plug it in again.

Unsafe removal of external drives (TM) has on a few occasions done this to me. The OS still thinks the drive is connected, so when you plug it in again, it gets confused. Mostly happened with power outages.

 

OP:

Is there any data on the old drive that is not already stored somewhere else?
What I'm trying to say is -- is the data on the drive anything you REALLY need anymore?

If not, I suggest you erase the drive (either on the Mac or on a PC), and then "start over".

You didn't tell us which version of the OS you're using (that's ALWAYS important to note when you post a question).

I would erase the drive to Mac OS extended with journaling enabled, GUID partition format.

However, I WOULD NOT use Time Machine.
I recommend that you give either CarbonCopyCloner or SuperDuper a try instead.
Both are FREE to download and try for 30 days.

Either of these will create a BOOTABLE COPY of your internal drive that you can just reach for and boot from in an emergency (such as an "I can't boot!" moment).
All your data is on it in "finder format", just as on any other drive.