Poorly dog

We woke this morning to find Seun, our 12 year old golden retrievr very poorly indeed. He couldnt stand up, couldnt even lift his head. Had to carry him to the car in a blanket to get him to the vet. Not sure what the problem is, high temperature, fast heart rate, completely off food and drink, and off his feet. Excellent service at the vet, seem straight away, various blood tests done, injections given and tablets too take home. He has managed to eat a small amount of rice and chicken, and now can stand up if you help him to his feet. Not keen to walk mare than 3 steps. Hope hes lots better in the morning .