In the event that partition table is corrupted or boot loader has gone, here are some steps that you can take in order to boot the system as normal.

I hope that you have taken the backup of partition table previously with the command;

Or if you also want to backup the whole MBR to a file do this;

From now on, I assume that you have booted the system in rescue mode with networking support.

1) The first thing is to paste this partition table back into its original space. Presumably, you have copied the partition table from network and put it under /tmp/sda_partition_table.out

This will fix your partition table. Now time to fix MBR

2) As the MBR has gone, we have to write MBR again. Do the followings;

The root (hd0,0) command assumes that your boot partition is in /dev/sda1

After these commands, you should be able to boot your system as before.