Connection lines, or Rats Nest (ratsnest), display is controlled in more than a few ways: The most direct way to turn on/off the display of all connection lines is to use the menu choice of the PCB editor:
View ► Connections ► Show All (or Hide All)
The connection choices can be used to show and hide all connections, specific nets, and nets associated with a particular component. Here's more information:
To affect Single Layer Mode (Shift+S), check this:
View Configuration (panel) ► View Options (tab) ► Additional Options (section) ► button for "All Connections in Single Layer Mode" on/off
If you are having trouble with connection lines, try:
Design ► Netlist ► Clean All Nets
If you've tried everything above, and your connections lines still are not visible there is one other thing to check. Open your PCB Panel and verify that the pulldown option at the top of this panel is *not* set to From-to editor. If the PCB Panel is in this mode, it can only display connection lines for the from-to you are viewing or editing.