Germany's hyperinflation in the early 1920's (and subsequent economic issues) were arguably a result of the treaty of Versailles. As part of the treaty, full blame for starting the war was laid at the feet of Germany, and punitive damages were levied in order to pay for the damage that was supposedly all Germany's fault. Initial demands totaled 266 gold marks (approximately $410 billion USD in 2015). The reparations were thought by many (including John Maynard Keynes, who famously stormed out of the Paris peace conference) to be unpayable. When it came time for the first payment to be paid, the German government quite literally just printed money. They did the same for practically all the subsequent payments as well.

Anyone with a basic knowledge of the price mechanism (an economic concept) knows that ceteris paribus, if there is an increase in supply of a good (the Reichsmark in this case) there is a decrease in price (the value of the mark). In summary, the destruction of the Riechsmark's value was caused by the actions of the German government. Whether they did this out of incompetence or desperation is up for debate.

I'm not sure what the German people used as currency in the meantime, but in 1924 (after 3 years of colossal inflation) the German government issued a new currency - the Retenmark - that was pegged to gold.

Interesting little sidenote - The German government did repay the majority of its war reparations in the end (not quite all, due to multiple interest writedowns). The final payment of $95 million was made in October 2010, after multiple interruptions due to things like Hitler outright refusing to make any repayments, and Germany being split in two.

Do you mean WWI or WWII?