Always in parallel - never in series.
You may have two or three small transformers - I had one on top of each cabinet with the 12V wire running down the back - they gave me no end of trouble and were either always failing, or intermittent.
Bad connections - and blowing halogens. The installers appeared not to have stripped the ends of the wires and being lazy had relied on the terminal screws to cut through the insulation to make a connection.
In the end I gave up and fitted small strip neons at £9 each - they never fail (a lie actuallly - one tube in three years) - less heat - better light and less running costs in power and replacement lamps.
And yes - a single bad transformer could cause all the lights to fail but I doubt that all eight lamps would be powered by a single transformer - it would need to be quite large at 200W or so - they would all work or all not work - no half measures with a single transformer.
Good luck...
Peccavi
March 2010