It's really important for guinea pigs to have food in their systems at all times, otherwise they will go into gut stasis, and eventually die, so eating is incredibly important. You need to get food into him ASAP. If you have Oxbow's Critical Care, that's the best thing for him (some vets sell it). If you don't have Critical Care, you can make a pellet mash. In a pinch you can also use vegetable based baby foods (I use Rafferty's Garden organic baby food), or blend up veggies and maybe grass to make a mash. It sounds like you'll have to hand feed. Do you have any syringes (without the needles of course) on hand?
In terms of what could have caused this, it's hard to say, but I would lean towards teeth perhaps. Do you feed plenty of hay? In any case if your piggy isn't eating it sounds like a vet visit is in order. If your piggy hasn't eaten since yesterday it's critical you get some food in him. How does his tummy feel?