Did you make the recovery disk as suggested when first starting partition majic if so use the boot disc.
From what I have read in the instructions partion majic removes the drive letter which hides the drive. I am not sure how you could re assign a drive letter without the PM recovery disc but some one may have the answer.
Although I can imagine accidentally hiding the partition I find it difficult for Partition Magic to hide the primary drive as it will always give a warning first. It would also be difficult for the drive to be completely cleared of partitions leaving 1 unallocated drive. (I could be wrong).
Reading your thread you state you installed a 160gb drive to recover a hidden partition. Have you removed this drive or have you accidentally removed/disconnected the wrong drive.
If both drive are connected, disconnect the 160gb drive check cables etc reboot with the paragon rescue disk and check the drive again.