What is wrong?

Shifting under load [especially while climbing] puts excessive stress on the drivetrain. Shift hard enough and you’ll bend or snap a roller. This appears to be the case looking at that outer plate.

Hard to tell from these photos, but the hanger looks intact and not even badly bent. Derailleur looks alright too, but it will need to be checked for play and function. When you take it in for a repair explain what happened to the mechanic. He’ll inspect all drivetrain components for damage.

As for a fix, it’s unlikely to break the bank. Likely work order as follows;

– Hanger Alignment
– Replace Chain [or new connecting rivet if not worn]
– Rear Derailleur Tune

In NZD this would likely be somewhere around the $70 mark with a new chain.

ETA: Bear in mind that sometimes there’s much more at play than meets the eye. The above is in an ideal world, assuming no other factors contributed to the event.