Go to the Control Panel / Adminstrative Tools / Computer Management. Then in the Storage section, click on Disk Management. Your other partition should be visible in that page. If it is, you can likely import or convert the disk to make it readable. If it isn’t listed there, then you partition information is likely corrupt and you would need to start over by deleting the partition information, creating new partitions, format the partitions, and then reinstall your OS and software. There is no real benefit to using multiple partitions any longer unless you just want “multiple” drive letters.