Had a 100km service for my Honda Accord Euro then it wouldnt start after 3 weeks.

I recently had my Honda Accord Euro serviced for it's 100km service. Within a few days, I could hear a small whirl coming from the engine when it was idle. This eventually got worse and then less than 3 weeks after the 100km service, the car wouldnt start today.

The dash showed that the battery wasnt charging yesterday. The RAA mans said something about the alternator.

1. Could they of done something wrong at the 100km service that caused this problem?

2. Should they of picked up this problem at the 100km service?

no
nothing could of been done, nor they would have done anything.
unfortunately, it's just bad luck.

Take it back to them lol and where did you get your ar service??

why would you take it back to them?
it's an alternator, they fail.
for the 100k service, they don't touch the belt.
or the alternator.
unless they have changed the belt, and you haven't mentioned it?

The problem is confirmed that I need a new alternator at a cost of $1277 !?!?!?!!?

:(

Try a wreckers in your area - second hand might be worth a go considering that price!
Also depending on where you got the price from an auto electrician may be able to replace the worn components - stator and/or brushes etc.