be blown off course

1. Literally, to be pushed off the route that one is trying to follow, as of a ship in the wind. Captain, do you know where we are? I think we've been blown off course! Has anyone made contact with that ship? In a storm like this, they are certain to be blown off course. We were blown off course earlier in our travels, that's why it's taken us so long to reach this port.

2. By extension, to have one's plans interrupted or interfered with. I just don't want your goal of becoming valedictorian to be blown off course by anything—including this new girlfriend of yours. Ideally, we wanted to save money for a house, but that was blown off course when Joel lost his job. My college plans were blown off course when I found out I was pregnant.

be blown off course

have your plans disrupted by some circumstance.

This phrase is a nautical metaphor: contrary winds turn a sailing ship away from its intended course.