I had dual boot ( windows 8.1 and Ubuntu ) in my laptop and after recent update got the error “unknown file system,grub rescue>”.
Steps to resole the issue
1. In grub rescue mode only ls set unset insmod commands
2. using ls (hd0,gtpX)/ , select the correct partition where you have the ubuntu file system. You might have to try this several times to select the correct partition. ( gpt6 worked for me , when the correct partition found , it will show lost_found, dev/, boot/…. etc )
3. set prefix= (hd0,gpt6)/boot/grub
4. insmod normal
Thats it 🙂