That sounds like the drive has been repartitioned/formatted and you’re now only seeing a small partition on it.
Personally for situations like this, I like Partition Wizard, which is a free (for home use) partition manager and toolkit which can view, change/resize and reformat partitions. Just be careful when using it, as of course if you get it wrong you can do more harm than good (double-check before committing any changes made, and ensure you’re doing them on the correct drive).
But used with care it’s a flexible and powerful tool - I often use it with my Raspberry Pi to restore SD cards which have been partitioned up for use there (Linux format dual partition). The program has good documentation, which I’d recommend you read. That said you can use it to view and analyse what state your drive is currently in before actively doing anything to it anyway.
