This can happen if the AW16G is powered off without using the proper shut down procedure.
You can boot the unit again by holding RECORD and MONITOR buttons during power up. ***WARNING, this will erase all song data. Do this procedure only if you have backup copies of your songs or are willing to lose your song data. ***
If you need to attempt to retrieve the data, contact Yamaha support. In some cases they can retrieve song data by having the user ship the entire AW16G to them.