At night you lose the sun (never!), but you will have the moon and the stars to guide you as they have guided travellers for centuries.
The moon, having no light of its own, is seen by way of light reflected from the sun, because the moon's lighted area varies as it waxes (grows larger) and wanes (diminishes), this, combined with its rotational rate, can give a rough help to your navigation. Cutting the technical explanation, if the moon rises before sunset, the light side of the moon is on the West. If the moon rises after midnight, the illuminated side is to the East.
As a rule of thumb, stars rise in the East and set in the West, but knowing, and being able to identify, some simple to recognise constellations will give you and even more accurate fix on your North/South line.

In the Northern hemisphere, we are lucky to have the Pole Star sitting over the North Pole with a variation of only about 2 degrees. This star is extremely useful for navigation, and can be found using the obvious constellations of the Plough (also known as the Dipper) (A), Cassiopeia (B), and Orion (C) (FIG.11).

The Plough and the Cassiopeia never set. While all three constellations seem to rotate around the Pole Star, Orion is the most useful if you are near the Equator.

An imaginary line drawn from the 'tail" of the Plough to the lowest of the points of Cassiopeia, goes through the Pole Star (FIG.12) while the central peak of the 'W' formed by Cassiopeia points to the Pole Star.

The two lower stars of the Plough also point to the Pole Star approximately four times the distance between the two stars. Orion rises above the Equator, and can be seen in both hemispheres (FIG.13) rising due East, and setting due West. The three stars forming the 'belt' point East/West.

Finding South in the Southern hemisphere is achieved by finding the Southern Cross. This is done by first locating a dark patch in the Milky Way (no, not the chocolate bar, the band of millions of distant stars) which is called the 'Coal Sack'. On one side of this is the Southern Cross, a constellation of five stars, while on the other side are two pointer stars. Make sure you do not get confused by other nearby cross shaped constellations (FIG.14).

By drawing an imaginary line five times the length of the Southern Cross's long axis along the same line towards Earth, you reach a point in the sky above the South point of the horizon. Mark a reference point here for your South point (FIG.15).

By setting up two sticks, or by using the sights of a securely positioned rifle, you can sight on a star to note its apparent movement over a few moments (FIG.16).

In the Northern Hemisphere: if the star seems to fall, you are facing West; if the star seems to curve flatly to the right, you are facing South; if the star seems to curve flatly to the left, you are facing North.
Although this is a rough and ready system, it can be made to work with any star other than the Pole Star. In the Southern hemisphere, the directions are reversed.
Should you have zero equipment, and 100 percent cloud cover day and night, the best advice is to sit and wait either for better weather or for a rescue party!