You can install the grub again. Boot from some LiveUSB (e.g. from Qubes OS installer in Rescue) and chroot your Qubes, assuming /dev/sdb is your Qubes drive:
Read how to mount your root:
How to mount a Qubes partition from another OS | Qubes OS
Mount your system:
mkdir /mnt/system
mount /dev/qubes_dom0/root /mnt/system
cd /mnt/system
mount /dev/sdb2 boot
mount /dev/sdb1 boot/efi
mount -t proc /proc proc
mount -t sysfs /sys sys
mount --rbind /dev dev
mount --rbind /run run
mount --rbind /sys/firmware/efi/efivars sys/firmware/efi/efivars
chroot .
If your ESP was deleted and you had to recreate it, then you will have to update it’s UUID in /etc/fstab. Use blkid to list the UUIDs of your devices.
Install grub and add EFI entry;
grub2-install --efi-directory=/boot/efi
efibootmgr -v -c -u -L QubesOS -l /EFI/qubes/grubx64.efi -d /dev/sdb -p 1