One of the smaller buttons to the right of the recon options controls the display of units (along with others for grid-lines, objectives, dead-pile)
You also have 'filters' that will only display certain unit types (corresponding to number keys - 1 for all), this is the left hand 'large tab' on the bottom of the map window.
If you have enemy showing, but no friendlies, the culprit is showing a scarce/absent unit type in the filter (Armour, Engineer the most common 'missing' types).
If you have lots of friendly, but no enemy showing - that could be what you have knowledge about, sometimes you do lose contact, but more likely you have 'current' intel set and most reports are over 15 minutes old. Try again with recent or all to get a stale but more complete intel picture.
Neither friendly nor enemy showing is usually unit display switched off - which is useful for terrain analysis, but seems to cause some confusion when all the units vanish unexpectedly. Hiding objectives and 'crosses' is also possible, again to get a better look at the terrain, but I find they are useful to define the likely next decision points.