I replaced a 20 amp double pole siemens breaker and my circuit is still dead. I have a submersable well pump and my shed on this circuit . It has been fine for 4 or 5 years and now nothing. I am a novice . Any ideas?

Did you check for voltage at the buss bar? How about voltage at the end, well or shed? Could be a broken wire

I have 110 on one of the wire at the well where it all comes together the other is dead

i have 110 on one of the wire at the well where it all comes together the other is dead

If you are sure all connections are tight, you most likely have a broken wire. I assume it is buried? If it in buried conduit you may be able to pull another wire through . How do you get 120 for the shed if you only have 240 going out.
You would need a neutral. Is this a 3 wire feed?