This is a really novice question but I'd like to change the gain on individual clips. Before I used to drag a little bar but I've since done something and it's disappeared and my timeline looks like this now .
Would anyone know how to return it to the previous?
You minimized the track heights, you can change track heights individually by dragging the line between the tracks with your mouse in the track list.
Learn also about the "hamburger" button in the "Vegas Pro Window" online Help (F1) item. It's the part starting "Access to the buttons that are not currently displayed can be achieved via the Hamburger button".
You could also create a volume envelope. but using the event's blue Gain bar at the top of the event is easier for a single event.
FWIW, the fastest way I'm aware of to add a volume envelope and change the volume of a segment is to use the 'V' key to show the envelope, highlight the segment you wish to lower (or raise) and drag the blue volume envelope line up or down. Envelope points are created by automatically.