Yellow Borders and Silence - help please.

After selecting dragging several apple loops onto one audio track of a new project, I have the following problems.

1. I get no sound on playback despite having no mutes or solos on. INitially it played 4 bars and then stopped - now it plays nothing.

2. When I select any region, there is a yellow border around it.

3. I cannot use glue tool.

Help appreciated.

I suspect I have inadvertantly enterd some sophisticated edit function