A hibernation file is stored in C:\hiberfile.sys if you have hibernation enabled. It contains parts of the memory at the time of hibernation, depending on the version of Windows. Run this to covert it to a raw image for further processing with Volatility.
volatility -f /path/to/hiberfile.sys --profile=<profile> imagecopy -O /path/to/output/folder/hibermemory.raw