Firstly, it's necessary to understand what "modernism" is. Depending on context, modernism can refer to an art movement dating from the late 19th / early twentieth centuries or to the general sweep of western thought dating from the Enlightenment to the present, as it is informed by rationality and rationalism (these terms are both general and specific - see dictionary). From early on, the culture of rationalism had its critics, but in the latter part of the twentieth century, the relationship between human existence and philosophical rationalism came increasingly into question. Post-modern in this context refers to what succeeds rationalism. In many ways, the success of rationalism can be seen in the 'advances' of science & technology, but in my view, the questionability of rationalism arises when it is over-applied to human situations. There were an increasing number of crises in the twentieth century and 21st century that have their roots in rationalism, yet remain little understood in these terms - for example, USA's loss of the Vietnam war; the post-communist economic re-structuring of Russia, the re-structuring of Iraq; the English education system and many of the day to day activities of governments and institutions that attempt to reduce us to a number or economic unit. The crises have their origin in the application of rational practices to arrive at a representation of 'reality' - the question is: if rationality gets us no closer to 'reality' (because of what it might omit), then where do we go? Ultimately then post-modernism is a question of representation: is rationalism any better than any other system of thought in modelling reality?
This is brief and therefore lacking in the subtleties; hope it is helpful in cutting through the language issues.