My ciabatta dough hasn't risen after 2 hours!
As it turns out, my dough was very cold. Due dilligence: here's a full answer.

One of the requirements of yeast for fermentation is appropriate temperature. Cold dough straight from the fridge won't rise, or it will only rise very slowly. 25-30°C is a recommended temperature range for rising, although cooler temperatures can work.

Duh.