Sometimes you can turn a large breaker off and it will never reset. I have had that happen several times.
Had this happen in a supermarket that we do work for. The building got flooded and the manager shut off the mains in a Square D I-Line panel. The breakers had never been turned off since they were originally installed. When we went to turn them back on after the flood waters receded, the internal mechanisms didn't want to cooperate. Fortunately, shortly before the flood, we had built a new service in one of their other stores and there were a couple spares for future expansion that we were able to use. Moving parts don't always operate well when they sit for several years.