Partition Loss
A drive that suddenly shows as "unallocated space," or with the wrong size or letter, usually still has its actual data untouched underneath — only the map pointing to it is gone.
What's actually happening
The partition table (an MBR or GPT structure) is a small map at the very start of a disk describing where each partition begins and ends. When it's deleted, overwritten by a disk-management tool, or corrupted, the operating system has no way to know a file system is there at all, so it reports the space as unallocated or empty — even though the actual file system and files a short distance further into the disk are completely untouched.
How recoverable this really is
The odds are usually very good here, since nothing about the data itself changed — only a small piece of descriptive metadata at a fixed, predictable location did. The real risk shows up afterward: Windows and other operating systems will often prompt to "initialize" the unallocated space and create a new partition on it, and if that new partition then gets formatted, its metadata can land on the exact same spot the old file system's own metadata used, which is a much harder problem to walk back from.
The general approach
Rather than trusting the missing or wrong partition table, recovery tools scan the disk directly for file system signatures — the recognizable boot sector or superblock patterns that mark where a partition actually starts. Once the real file system is found, the original partition boundaries can typically be reconstructed precisely, because most file systems store their own size and layout internally, independent of whatever the partition table said.
Don't let Disk Management, Disk Utility, or a similar tool talk you into "initializing" the disk or creating a new partition and formatting it. That prompt is one of the most common ways a genuinely straightforward situation like this one turns into a much harder recovery.
Practical, step-by-step guides for specific recovery software will be added here as separate linked articles.
This page describes the general situation, not a guaranteed fix for your specific case. If you'd rather have someone experienced take a look before you try anything, see what a hands-on evaluation looks like.