The fastest, easiest, most reliable "fix" in a case like this is to restart the missing drive, wait a good 60 seconds for the boot and network re-connection to settle, then try once again to access the drive. If that "old faithful" doesn't work, run something like
IP Scanner, or similar (there are several you can get free on the app store) to see if the drive is being
seen by your router. Let us know what information about the drive appears on the network or the drive becomes visible on the LAN.
NOTE: while it may seem obvious to you, a detailed description of the drive, the steps you take to connect it to the LAN, and those you follow to access the drive over the LAN. Could save hundreds (thousands?) of keystrokes and days identifying the underlying issue and devising a fix for it. I am not sure screenshots will reveal any information of value to this point, but if someone believes one is needed to clarify a point of understanding, they can certainly request you submit one.