Your rescue disc will also come in handy should you not even get as far as Grub loading. Once booted, verify the existence and state of your hard drive.
Open the file manager and see if your partitions are visible and if you can access the files on them – this is a good time to back up any precious files before you proceed further.
If nothing shows up, check whether the hard drive has been detected by opening the Disks utility from your Ubuntu Live CD – if you're using a Boot-Repair tool CD, you'll need to install the gnome-disk-utility through the Synaptic Package Manager under System Tools.
Once installed, open it via the Accessibility menu. The Disks tool lists all physically attached drives – if yours isn't visible, you may find the drive has failed, in which case you'll be reaching for your latest backup after shelling out for a drive replacement, or starting again from scratch with a fresh Ubuntu installation and new-found love of backing up your system.
Assuming your drive does show up, select it from the left-hand menu where you can examine the partition table plus check its physical health via its SMART attributes. Don't panic unless the drive is deemed on its last legs, but do focus your next check on the partitions themselves.
If you run the Boot-Repair tool, its recommended settings will include a full disk check, but you can manually perform this check yourself using GParted, which is on both rescue discs. GParted enables you to see how your partitions are arranged, as well as revealing which one is the boot.
Right-click this and verify Mount is greyed out before choosing 'Check to schedule a disk check using the fsck tool'. This will check for and attempt to repair any problems it finds as soon as you click 'Apply', but it's important the partition isn't mounted before the check is run. Also give it as long as it needs to complete – this could take hours or even days in some extreme cases, and cancelling or aborting will almost certainly corrupt the partition.
Make sure the check is run on all partitions on the boot drive. In most cases, assuming the drive isn't physically damaged or corrupt beyond repair, running these tests should ensure you're able to at least get Grub working again.
Take a fail-safe backup
It may seem strange, but if you're struggling with start-up issues, you should attempt to take a backup of your hard drive before you perform any repairs – this means if you mess things up completely you can always roll back your system to the state it was in when the start-up problem first manifested itself.
Of course, if you're diligent and you back up your system regularly, you could always simply roll things back now to a working state, although bear in mind there may be data loss involved if your home folder is on the same partition as your Linux installation (as is the case with default Ubuntu installs).
You'll need a suitable backup device – typically a USB-connected hard drive – and a tool that takes a complete drive image of your system. The dd command-line tool can be used from both Ubuntu and Boot Repair Tool live CD environments, but the backup drive needs to be at least the same size – and preferably – bigger than the drive you're copying.
At the other end of the complexity scale is Redo Backup & Recovery. You'll need a blank CD or DVD to burn its 261MB ISO file to, but it provides an easy to follow graphical UI.