Resize disk

Hi

I'd like to resize the raw disk from 50GB to 20GB. (VM OS - Linux)
I shutdown the VM, then I resize zith command:
qemu-img resize diskimage.zzz -- -30G

Then I try to start the VM, it starts but only in grub command line, so there is something bad zith partation or so...

How can I do it correctly? Maybe I forgot something?

Thanks.