If it's been gradually getting harder to start (and the temperatures have more or less bee the same) there's a whole bunch of things it might be: glow plugs, fuel filter, timing drifting.
If it's been starting really well and just this one time suddenly won't start I instantly suspect the glow plug fuse... on some AAZs it's bolted to the firewall, on some it's a big pink sucker at the top of the fuse panel, and on some (including mine) it's both.
If you have booster cables you can try a temporary bypass: use one cable to bridge from the positive battery terminal to one of the glow plugs for 30 seconds and then start the engine.