Do you have any other OS installed on your system which you can boot into? If yes, then for Windows OS, download and install MiniTool Partition Wizard Home Edition (or any other partition manager that allows editing partition flags), and in it, right click the partition you want to unhide (Windows 7 partition), go to "Modify" > select "Unhide patition".
For Linux OS, get GParted, and remove "hidden" flag of your Windows 7 partition.
In case you do not have any OS to boot into, get GParted Live by some way, burn the ISO to disk, boot your system to GParted Live disk, and remove "hidden" flag of your Windows 7 partition.