Main screen flashes on and off when connecting second monitor via HDMI

Hello,

I recently acquired a second monitor which I intend to connect as a dual head for my computer. As it was VGA - I bought a VGA to HDMI converter.

When I connect the monitor to my graphics card, it does not switch on at all. The main monitor then flashes on and off continuously.

The monitor connects perfectly to my brothers computer. There is something wrong my computer hardware/software.

Please help!