No problem - it'll still work. I don't use a yogurt maker - just a well insulated cooler and I have forgotten my milk lots of times and if anything the yogurt comes out a bit creamier (although that may be a function of draining it).

In fact, this last time I actually BURNED the milk and, rather than tossing it, I made the yogurt anyway. It's a bit dark and it has an unusually sweet flavor - think condensed milk - but it's edible.