Hybrid lock after fuel pump replacement

My 2011 Prius had the dreaded stutter/knocking at cold start-up. In order of repairs, in our own home garage, we cleaned the EGR valve (which was totally clogged), put in new plugs and coils for each cylinder, pulled out and cleaned the injectors, and put in sealant for a minor head gasket leak (no coolant in oil, just a wet engine below the gasket). The stutter got better after each work, but the car still lagged a bit, and sometimes shuddered with a cold start, but it ran so much better than before. So, we decided to replace the fuel pump and filter. That was the killshot. Right after putting in the new pump, the car ran the engine for a few seconds then went into "hybrid lock", whatever that means. We were able to start it again briefly enough to get a code reader to tell us we had a bad fuel pump. We replaced the new one with the old one (which worked way better than the new one before replacing it!), and it still won't start.

What went wrong? Can we fix this? All searches for "won't start after fuel pump replacement" bring us to things that have nothing to do with our issue.