Power system (PS) response to natural and man-related threats is subject to a large set of uncertainties regarding both the initial conditions of the system and the vulnerabilities of the Power and ICT systems to the threats themselves. This fact, together with the need to analyze high impact events (typical causes of blackout events), calls for the adoption of novel techniques to perform more in-depth security analyses. The paper describes a probabilistic risk-based methodology, developed within the European Union (EU) research project called AFTER (A Framework for electrical power sysTems vulnerability identification, dEfense and Restoration), which aims to perform risk assessment (including hazard, vulnerability, and impact analysis) of the integrated power and Information and Communication Technology (ICT) systems. Starting from threat/vulnerability identification and classification, and contingency scenario modeling, the paper focuses on the probabilistic simulation of the Power and ICT System response to the contingencies as well as on the computation of risk indicators for operation and operational planning purposes.