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All LED Flashing Simatic S7 400H CPU
All LEDs of both CPU of siemens S7 400H PLC started flashing while activating winCC of operator station.

There are 2 stations (1 operator stn & 1 Eng stn.) for PSA system. Operator station was out due to hard disk failure. It was made healthy by installing new hard disk & subsequently installing the software required like winCC, simatic manager for taking the PC in line. But after activating the winCC, the communication was lost & all LED's of both CPUs of PLC started flashing. Plant was non operational during that time. It was normalized by reseting the power, but the cause of such phenomena is not not identified. Diagnostic buffer is also not showing messages on that time.

What might be the reason for such event?

It's assumed from your brief description that the plant has dual redundant S7-400 processors with high integrity / failsafe I/O.

We can only guess at what happened - the best course would be to flag the issue on the Siemens Automation forum:

Yes, it was discussed with siemens engineer, but they also do not have firm root cause of such event. They have recommended to upgrade the firmware version.