Sporadic not starting in cold weather

Do you have the correct battery in it? Check to make sure the cold crank amps on the battery are correct, if they are try a higher rated battery

You have a defective valve lifter or lifters.

Check the fuel pressure, it could be a bad fuel pump

Cold weather has an adverse affect on batteries. The clicking sound could be from the starter solenoid not receiving enough voltage to energize the starter motor. The engine being hesitant to start and run could indicate the need of new spark plugs, wires, etc.

battery when you get cold wether the battery uses more power to crank the car over causing it to get week if its a diesel could be the heaster plugs hope this helps

READ THIS (happy ending so far...):
audi a4 2L turbo quattro, manual transmission with 98,000 KM. Perfect conditions, no oil leaks whatsoever.

Left the car off for two weeks in very cold weather (-10C down to -25C). When I restarted, it started immediately but with heavy rattling noise due to camshaft adjuster not getting enough oil pressure. After 3 minutes of warming up (outside temp.: -12C) at very low rpm (below 2,000rpm), I start moving the car. Still hearing heavy rattle noise. As soon as I pass 2,000 rpm, I get a scary red warning, low oil pressure. Turn off the engine immediately. In 1 minute, I re-start the car and the engine lights check ok, so I drive in first gear super slow (1,200rpm) hearing the same rattling noise. After another minute, finally the noise all the sudden disappears and the car starts running just fine.

Called Audi dealer and they told me that in extreme weather conditions, I should warm the engine for 10-15 minutes if necessary before start moving.
Take away: luckily, I didn't screw up the engine since I was using it at extremely low rpm (easy to do with clutch and manual transmission) until the oil reached full pressure. I am pretty sure now that the problem was leaving the car off for such a long time in freezing weather. All the oil flows back to the bottom, and when I restarted it, the cold oil with high viscosity took a long time to reach all the parts of the engine, specially the camshaft adjusters that are actuated by using oil pressure. Next day, the car started at -9C perfectly without making any noise (as usual). Basically, the longer you leave your audi in cold weather, the longer you might hear the rattling noise until the oil reach full pressure inside the engine. My car took 6 minutes to fix it after couple of weeks in cold weather. This makes sense to me; in fact, in the past I only heard the rattling noise after leaving the car off for a couple of days in the winter.
So far, this has had an happy ending....

Its likely having problems because of how cold it is. A block heater is beneficial during these times. I recommend getting some starter fluid. Have someone spray it into the intake while you try to start the vehicle. This should get your engine back up and running, if not you may have water in your fuel from the cold weather. You can also try disconnecting your fuel supply lines from the cylinders and trying to crank the car. Once an observer sees fuel flowing in steady spurts reconnect the lines and try starting it. Good luck!

Hi, there could be several issues that could cause this, When the cold weather strikes everything changes, Oil thickens up and if there is water in the fuel it can freeze and no fuel is being pumped, Also it could be a mechanical issue now that was just intermittent and now has stopped working completely, like you said the car wouldn't start 3 times this week and now nothing at all. Your best bet is to take it to a local garage and let them take a look at it, when it comes to starting there is a lot it can be. They have diagnostic machines that can check all the sensors electrical on the car and narrow down the issue a lot faster than trying to figure out what it can be by replacing part by part..

well will it start using a little shot of starting fluid ? if so you might have a weak pump . try this and see . thanks Jerry

Clean & tighten connections at battery is a good place to start.