If you can't boot the system you may want to use the gparted linux distribution instead which includes testdisk and can be booted from CD (recommended). Just download the version of Testdisk that is offered for your operating system and execute it. Tip: if you believe that hardware errors are the cause, try the following system rescue CDs instead: Trinity Rescue Kit, the Ultimate Boot CD or Hiren's Boot CD.

Testdisk will only help if the errors are software based, if you encounter a problem that is hardware based you are out of luck. Testdisk, which is an open source data recovery live CD, may be the tool you need when you encounter a situation like the one described above. While it may repair corrupted files for example or the master boot record, it won't resolve hard drive errors. That is often not the best course of action as you may or may not fix the issue doing so. If you do not know what caused this and do not know tools that can fix this your only option is to install the operating system again.