I had a wrong fstab generated by genfstab (as pointed out here). So the kernel (please correct me, if this is wrong) didn't find my root-partition.
I generated fstab with labels and had a partition with a space in it. In fstab this must be written with \040. genfstab wrote garbage for the space.
Other answeres suggest to run update-initramfs -u -k version, however this command is replaced by mkinitcpio.
To get the system running I did this:
I ran
grub-mkcofnig -o /boot/grub/grub.cfg(probably not important in this case)after that I booted into grub-menu and pressed c for the grub-shell I started Arch Linux manually by entering the following commands:
insmod linux insmod ext2 (enter your filesystem-driver) set root=(hd0, 2) (enter your partiton-number starting from 1) linux /boot/vmlinuz-linux root=/dev/sda2 (again select your partition) initrd /boot/initramfs-linux.img boot
correct
/etc/fstab
Finished!