I guess you need to do a boot repair. All you need is the same bootable flash drive and follow instructions online as to how to repair GRUB. This kind is think

I guess you need to do a boot repair. All you need is the same bootable flash drive and follow instructions online as to how to repair GRUB. This kind is think l thing happens a lot, happened with me too. If the problem persists, contact me, I'll mail the steps.