Can’t mount external drive - Mojave
From Terminal, what happens if you try:

sudo diskutil repairVolume disk2s2
sudo diskutil mount disk2s2


This will run the Disk Utility disk repair process. Can you report back with any errors or output you get.

You can also try booting into Single User Mode (at start-up hold down Command-S). That'll drop you into a command-prompt and you can try

sudo fsck_hfs -fy /dev/disk2s2
shutdown -r now


That runs a filesystem check (hence fsck), and can also repair damage, then reboots. After the reboot, check and see if it mounts.