"Instrument not found" error message

I played back a Cubase 8 Pro orchestral file using Miroslav library with no problem, then shut PC off (Win 7).
Later that week I booted Cubase again and the same file and started getting error message “Instrument(s) not found” for each channel.
I do not think I touched any setups or erased anything or changed paths, etc. in the PC before getting these error messages.
How do I trouble shoot this ?