getting windows shadow copy error when running bak of partitions

Have 2011 acronis. Win 7 64 ultimate Tried to run acronis 11 bak overnight to bak my c and d partitions. woke up and found a windows error: Windows backup failed while trying to read from shadow copy on one of the volumes being backed up message showed the location of my windows bak, not the location of the acronis bak job I'd just run...