Check your line font colour is not the same as the background.
Check the colour depth is set to at least 32 bit.
Check your graphics drivers is up to date.
I just encountered the same problem on an existing file. The file opened correctly several months ago, but today for some reason some (not all) of the lines were not visible. They were selectable, and when selected they show up, but as soon as they were unselected they were no longer visible. Graphics drivers up to date, color depth 32 bit, change layer, change linetype, change color, change background color, nothing met with any success. Finally saved it out as a R12 dxf, open the dxf, and all is well again.