Reason is not sufficient on its own, as it does not contain in itself its own principles. Reason is a dialectical and indirect way of knowing, and when it's limited to empirical data it can only proceed by analysis, which cannot arrive at a synthesis due to the indefinitude of possible empirical data. In addition, proof isn't the same as the thing to be proved, as that would amount to the two being identical, therefore sensual experience and discursive reason can only indicate and point to certain higher things, which are the most essential of all. This doesn't amount to advocating for irrationalism but rather indicating the necessity of supra-rational knowledge, which is the intellect in its traditional meaning, although one can use it just as well by accepting the foundations that are given to him on authority or which are implied in common usage.
Modernity started out from a position of a relative certainty deriving from accepted norms, but by cutting itself off from religious and metaphysical foundation it eventually replaced them with purely human philosophies and attributed absolute value to things that were contingent (which is not to say arbitrary, but simply founded on something higher still), making itself vulnerable to extreme relativism and other kinds of critique grouped under the ‘post-modernist’ designation. Limits imposed on knowledge simply made it plausible for deconstructionist thinkers to deny objectivity or validity of all normal assumptions which people used to hold, on which cultures were founded, and on which sciences hinged.
One example of an early modern assumption that opened the way for the present chaos was the new understanding of freedom, which, following on nominalism and individualism, posited that to be free is to lack any constraints on one's will and to be completely spontaneous on one's own terms, as opposed to the traditional understanding of freedom as the perfect conformity to human nature and not being under the sway of error or sin.