Yes I have the same problem with my library on my Synology NAS.
The Nas goes to sleep, then ITunes says it can't find the library.
I have just changed the NAS settings to stay running for 4 hours before going to Idle mode when not in use, that at least should reduce how often this occurs.
I also have the NAS music folder in my Login Items in System Preferences.
I am even cosidering scheduling the NAS to be Up from 08:00 to 24:00!
Why cant iTunes take account of it being offline and wake it up!
It is definitely a shortcoming, when so many of us now wish to offload music and photos etc., to external drives.
I had the same problem with iPhoto.
Don't forget the comment from Limnos in an earlier post:
"There's a difference between moving your iTunes media to a different drive (reference in another post) and moving your whole iTunes library".
Your 'iTunes\iTunes Media' folder is where all your songs are held - iTunes\iTunes Media\Music.
This is shown as the 'iTunes Media file location' under iTunes Preferences\Advanced.
The iTunes 'Library' is the 'iTunes Library .itl' file in the iTunes Folder, this is what you can change by holding down the Options key when startin iTunes.
By the way does anyone know a way of seeing the library iTunes is currently using just to check?