Troubleshooting dropped phase in 240 volt line
To determine if there is a break.


Turn off the breaker in the main panel.
Remove the conductors from the breaker, and connect them with each other.
In the barn panel, disconnect the ungrounded  feeder conductors.
Test for continuity between the ungrounded conductors in the barn.


If the test shows open (no continuity), then there's a break in one of the lines. If not, then something else is wrong.

If the lines test as not broken, test for continuity through the feeder breaker.


With the conductors still disconnected, turn the breaker in the main panel on.
Test for voltage at each of the terminals.


If they both show voltage, the breaker is not the problem.  If either does not show voltage, replace the breaker.

If the lines do test as broken. You can either dig them up and look for the damage, replace them (abandoning the existing cable), or contact a local Electrician to see if they can locate the break for you.