Short answer: the reproducibility crisis is the concern that many scientific results are difficult or impossible to reproduce or replicate, especially when studies use small samples, flexible analyses, poor reporting, or weak incentives for verification.
Reproducibility and replicability are related but not identical. Reproducibility often refers to getting the same computational result from the same data, code, and methods. Replicability usually refers to obtaining consistent results in a new study or independent test. Both matter because science depends on results being checkable.
Why it happened
The crisis did not come from one flaw. It came from many. Researchers may face pressure to publish novel findings. Journals may prefer positive results. Methods may be reported incompletely. Data and code may be unavailable. Small studies may produce unstable findings. Statistical significance may be misunderstood or overvalued.
Some fields have been more affected than others, and not every failed replication means the original result was false. Still, the crisis exposed weaknesses in how research was produced, published, and rewarded.
How science is responding
Reforms include preregistration, registered reports, open data, open code, better reporting guidelines, larger samples, replication projects, and stronger statistical training. These practices make research more transparent and easier to check.
The reproducibility crisis is often described as a threat to science, but it is also evidence of science correcting itself. The problem would be worse if researchers refused to notice it. A mature scientific culture should be able to inspect its own methods.