How to open my external hard drive on MacOS?
There are many reasons why a volume won’t mount: How does Mac OS X decide which volumes to mount? One of my disks doesn&#39;t mount at boot

Since yours shows in disk utility, I would either run a verify or repair operation based on if the data is backed up elsewhere. This could be the first sign the filesystem or hardware are failing and when you “repair” a filesystem, sometimes that deletes bad files or causes other data loss. 

It could also be that the filesystem check is taking hours and that given enough time, it will mount again rapidly once a full check is done. You can see those check details in the disk utility or possibly in /private/var/log while the checks run.