You've probably entered Expanded Edit Mode by inadvertently double-clicking on the edge of a video clip - it's quite easy to do without realising. The edge of the video clip which was double-clicked should be red. The simplest way to get back to your original timeline display is to double click again on the red edge of the video clip and then wait a few seconds while Vegas re-aligns itself.

You can check out more about Expanded Edit Mode in the manual if you want as the manual gives more ways to enable/disable/use Expanded Edit Mode.

BTW, it is disconcerting when it accidentally happens.