Please post the information below and try the possible solutions mentioned at the end.
What you were doing
Please include when this error occurred, and what you were doing at that time. (A screenshot would be helpful)
Please include, your RAM, Graphics Card, CPU, and CPU/GPU Temperatures. For any other issues, post the details on the hardware giving you trouble.
(To find most of this info, Start > Run > dxdiag)
Please state what version of Windows you are running, including 32 or 64 bit.
Please post a screenshot of your video settings in HoN (only necessary if you have graphical issues or crashes).
Possible Solutions (in the given order!)
1. Update your video drivers (nVidia / ATI / Intel)
2. Update DirectX
3. Remove Heroes of Newerth completely (this step includes deleting the HoN-folder under Documents!)
4. Download the latest client and install it again.