Hi, I've had a guinea pig who had a stroke at 2 years old as well - and it sounds exactly the same as your guinea pig. Her name was Zara, and it was a very traumatizing experience for us both. When I went to the vet they gave her some pain medication for her eye and a cone (like the ones the give dogs to stop scratching). She hated the cone and always had one paw stuck on the neck! It helped her eye from being scratched up more though. They also gave us critical care and we fed her through a syringe three times a day (make sure to monitor their poops closely, if they aren't coming out like normal you should feed them more). Her eye was clouded over, we were scared she was blind too, but don't worry this is a natural response when guinea pigs eyes are injured. The vet told us she had been rubbing it against the ground since she'd been lying on her side and this had caused a small lesion [I forget what the technical term is] to form. The vet also gave us an eye cream which we put on with a cleaned finger, it had antibacterial and inflammatory properties.
I know that it is terrifying - but do not lose hope! There isn't much one can do for strokes, but this doesn't mean this is the end. Over a week or two my guinea pig began to regain control of the half of her body which had been unresponsive. To encourage exercise we led her across the floor with a grape, and sure enough she began to get stronger. It also put my mind at ease since at least she wasn't in any pain (other than her eye but the eye medication solved that). Her eye even began to clear up.
It takes a while but I'm sure your guinea pig is strong. Make sure to see a vet ASAP so you can get all the materials you need.
I wish you the best of luck and if you ever need to talk to anyone who has been through this, I'm here.
P.S. The vet also checked for an ear infection because sometimes that makes guinea pigs lose all sense of balance. It's possible your guinea pig could have this, which I believe is easier to treat with a course of antibiotics. Not too sure though!