240 v. Problems to the other garage
Open hot in the garage feeder
What you are seeing points to a break, or open, in one of the hot wires going from the main panel to the garage.  With the all-too-common setup of a directly buried cable, this isn't all that uncommon a failure mode.  I'd first turn the breaker going to the garage off and undo and redo the terminations on both ends of the cable with an inch-pound torque wrench set as per the labeling on the devices.  If retorquing the connections doesn't fix the issue, then I'd be suspecting a broken underground wire, say due to a rodent chewing it.  An electrician might be able to locate the break with special tools, but otherwise, it requires replacing the feeder outright, preferably with individual wires in a buried conduit, which isn't as likely to get gnawed to bits.