Do you have NVidia video card? If yes, you are affected by this ancient bug:
http://forum.thegamecreators.com/?m=forum_view&t=158209&b=15
First of all, update your video card drivers. I've had that problem for a long-long time but with the latest NVidia drivers, it does not happen any more, it seems to be fixed.
If the driver update doesn't help (but it should), then place your drawing commands into a game loop. Are you using the Gaddis book? I've seen some people posting code from that book and it seems to start exercises by one-time drawing onto the screen, without game loops. When you create a new Dark GDK project from the game template, you can see that there is a while loop which keeps the drawing commands continuously repeating. This may solve the issue:
#include "DarkGDK.h"
void DarkGDK ( void )
{
dbSyncOn();
dbSync();
while (LoopGDK())
{
// put all drawing commands inside the main loop
dbLine(250,210,320,280);
dbCircle(308,105,50);
dbSync();
}
}
If that still doesn't help, then look for alternative ways for drawing circles, instead of dbCircle and dbEllipse. Here is one workaround option, which creates circles/ellipses from short line segments (it's ugly but at least you can see something on the screen):
http://forum.thegamecreators.com/?m=forum_view&t=158250&b=22
I have seen this plugin recommended too, but I haven't tested it so I don't know how well it works, or whether it still works with the latest version of Dark GDK:
http://forum.thegamecreators.com/?m=forum_view&b=5&t=69221&p=0
I hope one of these steps will fix your problem.
(P.S. When you post code, then please put it into code blocks. It doesn't only make the post shorter but it preserves the special characters and indentation as well.)