Go to the OVH manager
1: Select your dedicated server
2: Click on Services
3: Click on Netboot
4: At bottom select hd (Hard Disk) from netboot option
5: Reboot the server
Once you reboot the server, the server will boot from the hard disk.
If still it does not boot from the hard disk, contact OVH because they probably have locked your server which is preventing you from booting it via HD.
Also, when OVH sends you Anti-Hack emails it does not necessarily mean that your server has been hacked. It means your server is abusing their TOS.