My Cat Has Vomited White Foam 3 Times Today. Not Drinking Or Eating Or Moving Around Much. What Could This Be And What Should I Do?

This white foamy vomit has only happened this one time. Cat is Maine Coon and is 2 1/2 yrs. Old. He hasn't ate any food or much water today. We have been using an eye dropper to give hime small amounts of water. He is house cat. Never outside in the yard. Please help!