Blockchain technology, with its transformative potential across industries, has ushered in a new era of decentralized systems. However, its widespread adoption has exposed vulnerabilities at various layers of its architecture, posing significant challenges to security and integrity. This paper introduces a comprehensive layered framework for blockchain security, classifying threats across five architectural layers: Application, Contract, Consensus, Network, and Data. By mapping vulnerabilities to these layers, the framework highlights specific attack vectors, such as Reentrancy, Sybil, Selfish Mining, and Replay attacks, and provides targeted mitigation strategies. Furthermore, the paper examines the disruptive potential of quantum computing on blockchain security, emphasizing the need for post-quantum cryptographic solutions to future-proof blockchain systems. The classification and analysis aim to guide researchers and developers in enhancing blockchain robustness. The findings contribute actionable insights into securing blockchain ecosystems and charting future research directions, including addressing interoperability challenges, optimizing smart contract security, and strengthening consensus mechanisms against evolving threats.