From MEPIS Documentation Wiki
If you damaged your OS loader and the computer doesn't boot anymore you could try to reinstall GRUB:
- Boot the MEPIS CD.
- Select the resolution that work best for you.
- Log in as root (password is "root").
- Start MEPIS System Assistant
- Select Repair System Boot tab.
- Reinstall GRUB in the same place you put it when you installed MEPIS first time.
If that doesn't work try the command line:
- For MEPIS 6.X
grub-install --root-directory=/mnt/hdc1/boot/grub hd0
- For MEPIS 7.0
grub-install --root-directory=/mnt/sdc1/boot/grub sd0
(this assumes that you install GRUB in MBR of the first hard disk)