TestDisk is a powerful drive and data recovery software that is designed to help recover lost partitions and/or make non-booting disks bootable again. This can be caused by faulty software, certain types of viruses, or of course human error (such as accidentally deleting your partition table).
The program is cross-platform with recovery tools for a long list file systems and includes the also-portable recovery software PhotoRec.
Note: Use with caution; it’s recommended that you read the documentation and take your time. The software can easily made dramatic and difficult-to-reverse system modifications.