How to navigate an aircraft without GPS?
The question you should be asking is rather, how did airliners navigate before the widespread adoption of satellite-based navigation systems such as civilian GPS?

One can navigate by way of landmarks (as is typical in low-altitude VFR general aviation), but more typical in commercial aviation in the absence of GPS would probably be to navigate by either inertial navigation, or by radio beacons. There are or have recently been several systems that can be lumped together as "radio beacons", such as for example NDB, VOR/DME and ILS, which are used for different purposes, and have different strengths and weaknesses. Until recently, another such system was LORAN.

Major airports typically offer a number of radio beacons that can be used for locating the airport, and there are normally waypoints, sometimes associated with radio beacons of their own, along which a route can be flown even without external references (such as in instrument meteorological conditions, IMC). 757toga's answer to this question shows what such a navigation system for long-distance flight can look like.

GPS does make it easier to fly the most efficient route possible under current conditions, but it is hardly required in order to fly some route from one airport to another. Being prepared to fly without GPS might, in the simplest of cases, simply mean that you plan for a slightly less direct route and treat your non-GPS navigational aid equipment as required, possibly even primary, rather than secondary or nice to have, since you know there's a clear risk of losing GPS through no fault of your own or of the onboard equipment.