How to find grub version because i can't able to boot my computer
I believe Red Hat still uses Grub Legacy. Do you have a Linux Live CD or Linux on a flash drive? If you do, boot it and create a mount point for your / filesystem partition, mount it and find the grub files. I don't use Red Hat so don't know where they would be. As I said above, they are usually in /lib/grub or /usr/lib/grub/ directory. You would then need to copy them to your /boot/grub directory. If you have a separate partition for boot, you will also need to create a mount point for it and mount it. I expect you will need to create your own grub.conf file unless you have a backup somewhere.
I don't know why you would get a 'no such file or directory' message for /sbin/grub or /sbin/grub-install although I would not expect either to work in your circumstance.