Well it would be helpful if you listed your system specifications. Based on the very limited information you have provided I am assuming that your HDD is losing contact with the controller at a BIOS level. When you reboot then it establishes communication again. Here are three possibilities.
1) Your Motherboard needs a BIOS update
2) Your HDD needs a Firmware update
3) Both of the above
Also if you have a Gigabyte motherboard I have noted that there are issues with the HDD losing contact with the SATA controller. I fixed this by RMAing the board. There is some issue with the south bridge on certain Gigabyte boards in my experience.
The HDD will work fine for a while so that has been noted. Mine did not show problems for almost two years.
More likely to me it may be a construction defect of the HDD that is now becomming apparent or the drive may also be overheating. It may have a failsafe that shuts it down and it get's reinitialised upon reboot. Check if it gets very hot.