I'm afraid the MFT/FAT (depending on the file system) of the partiton may be cleared, overwritten or corrupted.
If this happens, the partition seems there - but the file system is not readable.
I'm afraid in such situations, Hard Disk Sentinel can't help, as it does not perform recovery of files/file systems.
If you have drive letter assigned to the partition (eg. D: ), then chkdsk D: /r may fix the partition, as it may be able to process and repair the file system.
But be careful, as in many cases, chkdsk can cause even more troubles (can even cause loss of many files in the partition) so you may try to ask a data recovery specialist - or make a full sector-sector clone first (before attemption chkdsk), just to be sure.