Wake a sleeping trade convoy

The Trade Convoy should be at whichever city it was based at, though selecting it can sometimes be a little difficult if you have a fortified military unit in the same tile. I find zooming out can help with this:

You can then select the trade unit circle, which will bring up the unit's action menu. From there it's just the usual process of selecting the Establish Trade Route option or the Wake Up clock:

If you've lost the trade unit completely, you can access it via the Military Overview menu, as Dirty Beach describes.

While Tim's answer is correct, there is certainly an easier way: open the military overview.
"Military overview" is a bit a misnomer in my mind. It's really just a list of your units.

You can open the military overview by clicking the '+' on the bottom bar, then selecting "Military Overview" from the options that appear, or by pressing the F3 key. Civilian units (Workers, Colonists, and Trade units) are at the bottom of the list. Find the unit you want, click it, and it will select it for actions. Close the overview, then issue commands to the selected unit.