In energy harvesting applications, a power supply generated from a renewable power source is unstable that may induce a sudden power outage, losing data being processed. This paper introduces a sudden power-outage resilient nonvolatile microprocessor based on a flashback architecture for immediate system recovery, where nonvolatile storage elements, called magnetic-tunnel-junction (MTJ), are exploited with standard CMOS gates. In the proposed architecture, data stored within a past few clocks are also kept in redundant MTJ-based nonvolatile flip-flops (NV-FF), while storing the current data in NV-FFs. Even if a sudden outage induces a failure of storing the data partly, the inconsistency of the stored data is automatically and immediately recovered by reordering the past data, which enables a continuous operation. A nonvolatile Cortex-M0 processor based on the proposed architecture is implemented using 90nm CMOS/100nm MTJ technologies. It demonstrates the immediate system recovery after a sudden power outage with a 32% area reduction compared to a triple modular redundancy based equivalent microprocessor.