Hitler's fantasies of conquering all of northern Europe were deranged. He would have to defeat both Britain and the USSR, each of which have an economy the size of Germany. Even if he got Italy on his side, that just offsets the huge advantage France and Russia had against the Germans.
My best-case scenario for the Natsocs would be that France and Britain let Hitler negotiate for the Sudetenland, including all of its forts. Hitler decides to not stop in the Sudetenland and conducts a false flag operation with one of the forts, triggering war between Germany and Czechoslovakia. France and Britain for some reason decide to just let this happen. France isn't too hard to convince, though, as in 1938 the political situation was very anti-war. Hitler takes Czechia and gives Poland Slovakia in exchange for Poland's neutrality. Hitler then conducts a surprise attack on the Netherlands and Belgium and thanks to the additional materiel from the large Czech war industry, he defeats the Dutch easily and moves all the way to Flanders. The Belgians didn't let the French army into their country before the war started, so the French army is still largely in France. Funnily enough, the French OOB in 1939 did not allow for a strategic mobile reserve. They assumed lines would hold. If Hitler's sneak attack gets all the way to Picardy, you could see the French try to shorten their lines by retreating out of the Ardennes in Belgium. Considering the impulse to try to defend their own land, it's not that much of a stretch. Imagine a German encirclement of a significant French force in Picardy and it might be open ground to Paris, like in 1915.
So Hitler has defeated Belgium and the Dutch, and encircled a French Corps and opened the way to Paris. What now? He sends a token company to drive around in Paris and then back to his lines, because going all the way to Paris will open him up for a counter encirclement. The French lines redeploy and congratulations Hitler, you recreated WWI, but this time Germany has no allies.
If anyone doubts the German army could move that fast, you're largely right. The German army wasn't as mechanized as the French one. What allows them to move so fast is that they have a corp of tank divisions, much like the Soviets. People forget that the pre-communist Heer actually had a very forward-thinking high command. They didn't treat tanks like cavalry, but as a way to maintain high tempo operations on a strategic level. Their individual tanks may have been worse, and their infantry divisions were worse than the French, but in any battle of German vs French tanks, the Germans would have a significant numbers advantage. Combine this with the fact that the Germans had radio in their tanks and the French still used flags means that the German Panzer divisions had significantly more strategic and tactical mobility than the forces they would have faced. Assuming that the Germans don't fight the Soviet Union, of course. Problem is that they had very little to follow it up with. By the time they could get their main lines to exploit the encirclements, the French could use their interior lines to redeploy and then there's not much the Germans can do after that, as the French have a more robust economy and an ally with a large army and even bigger air force and navy just across the Channel.
The German economy would probably collapse after a year or maybe two of fighting. It's Germany so I wouldn't underestimate their ability to produce men or materiel. The problem is that the "Third Reich" was largely financially insolvent even by the time of Hitler's death. They would have run out of specie to trade with by 1940 and then Sweden and Switzerland will stop operating as middle men for the Germans. The Germans would run out of rubber and oil, they would even start running out of food, just like in WWI. People would remember the economic hardships put on them by the Kaiser and see their Fuhrer did the same and the Heer would most likely mutiny. In addition to assassinating Hitler, the army would most likely just storm the Reichstag and announce a military dictatorship to bring peace. We could see another stab-in-the-back myth, but if the military is leading the peace negotiations it would be hard to gain purchase.