I am having a similar issue with my son. He, too, is almost 5. He was in Pre-K this year, and he will go to Kindergarten next year. Just in the last month, he has developed a fear of the dark. It was so sudden that I think something happened or was said at school or day care to set it off for him.
I found the following information on MentalHeath.net under Child Development and Parenting. This was part of an article about preschoolers. It made me feel better about my DS and how we handled his new fear.
"Fear is another strong emotion that many children experience intensely during the early childhood stage. Because children at this age often have very active imaginations and are still learning the difference between reality and make-believe, they are very susceptible to strong fears. In particular, young children often have intense fears of lightening, thunder storms, monsters under the bed, or other scary dangers their minds create. Many children also experience nightmares, or bad dreams during early childhood.
Parents can help young children by offering some extra emotional support in a way that doesn't ignore or minimize the true fear their children are experiencing. For example, caregivers can put a night light in the child's room to help lessen shadows and the monsters that may lurk in the unlit bedroom corners at night. After a nightmare or during a storm, caregivers can offer support and physical comfort such as hugs.
Caregivers can also foster autonomy and confidence by helping their kids think of creative and lighthearted ways to deal with fears. Children can be taught to use a simple prayer or silly song to help them feel safe and in control of the situation. For example, Grandpa can play a game with Jimmy at night to "spook the closet monster" by shining a flashlight into the closet as part of the bedtime ritual in order to exterminate any ghouls, goblins, or sock-eating creatures. Jimmy can laugh, but also feel a sense of pride that he is in charge of the situation."
Hope this helps!
M.