This happened once before & I called a plumber but after 4 days it stopped & when the plumber came he said he couldn't do anything because he couldn't see anything. Can I fix this myself?

If the water is dripping off the cap on the fill valve, I wouldn't worry about it too much, as long the toilet is filling properly, and the fill valve is shutting off properly. A little dripping from the cap is somewhat common. Depending on the fill valve you have, you can just dissasemble the top of the fill valve (with water to toilet shut off) and clean or replace the diaphram, or just replace the complete fill valve (fairly easy and cheap to do).