There is 120 volts on the metal box for my breaker box The box is 100 amp. What are some reasons for this to happen.
First, shut main power off and use a big wooden stick to do it. Seriously.
Finish reading this on your phone. Every ground in your house is hazardous.
Then, re-test to confirm the problem has in fact gone away. If the problem is still there, then call the power company to report an outage, tell them what you are getting and there should be a truck out there within a few hours. (That last promise void if your power company has recently experienced a weather calamity like hurricane, flood, or ice storm.)
OK, the problem is gone. If you can set yourself up a safe environment to do this, and a reasonable testing regimen, you can turn off all the branch breakers, and turn on the main breaker. Then, turn on branch breakers one at a time until the problem returns. You're gonna want to use the broomstick for all this. If a single branch circuit causes the problem to appear, note that down. This is not critical to fixing the problem, so don't take any undue risks collecting this data. But it will behelpful information later, this circuit has a ground fault.
Now with everything off again, and the panel testing safe: off to the library you go. You need to pack your brain with all you can about the
Grounding Electrode System
This actually is a fairly simple beast, but its few parts must all be correct. And "beast" is the operative word, because a stoutly built grounding electrode system prevents this from happening by brute force.
The starting point is something that ties to Earth: either grounding rod(s) that are (current spec) 8 feet long, and 2 rods (unless you do a test that costs more than another rod); a water main if it can be assured it's all metal (and if you're in Flint, it may not be since so many water supply pipes are being replaced with plastic), or a purposefully constructed UFER connection to your concrete slab's reinforcing rod network (by far the best, but nobody ever remembers to spec it.)
Then you have some nice, beefy ground wires (e.g. stranded #4) from the electrode to the main panel. This ground must be one continuous run, it cannot be cut and spliced. (But can be copper-welded ie brazed).
At the main panel end, it firmly attaches to the metal of the service panel, and/or the grounding bar. I am fond of belt and suspenders, I use a thru lug on the panel proper, and then onto the grounding bar. This is what assures the equipment grounding system (panel case and all those green and bare branch circuit wires) cannot be far from earth potential.
The above system is the one that has failed for you. As you can imagine, it is not a sexy or attention-demanding system, and is oft-forgotten.
Look for anything changed or out-of-the-ordinary. The classic is the water company fitting a smart meter that happens to be plastic.
Now is the time to give that system a thorough inspection and fix/overfix anything that is deficient or not up to the latest codes. You want this stuff beefy. In a case like this, there can be a violent tug-of-war. It's important for safety that the grounding electrode win and the branch circuit breaker be forced to trip. If the branch circuit wins and kills the grounding electrode, then you get this failure mode.
Once all that's done, then come back 'round on the ground fault issue on the bad circuit you identified. If you did a good job here, it will trip the breaker instead of electrify the house.