MacOS High Sierra - Unable to mount network drive after waking from sleep
We have a similar issue with our network drives at work. When that happens the following works:

Hold down the OPTION key. 
Right click on the finder icon.
Left click on RELAUNCH. 

Doing this seems to clear out the cache. Then the system realizes that while it was sleeping, it did in fact lose the connection to the drive, and lets you reconnect.