Remember that mummy (or daddy) have magic powers and can "whoosh" away monsters with a special puff from her mouth and flick of her hands.
You can also roll up any bad dreams or monsters into a tight ball (about the size of a football), throw it out of the window and a lorry will drive over it and squash the monsters into squidgy monster jam, which will wash away.
These are two strategies that we use with our children, the first works better for the younger child, the second works better for the older child.
It's probably because of the excitement of starting nursery, but over-tiredness and over-stimulation from films can make it worse. It's surprising what can scare children - ds used to be petrified of one or two particular moments on Thomas the Tank Engine!
Dd hates it when she can see the glowing little LED on an extension lead that we use in their bedroom. It really spooks her in the dark room, so I have to make sure the extension lead is well tucked under so that she can't see it.
I, OTOH, make sure never to leave clothes draped over furniture at night, as I half-wake and think that I'm seeing a person in my room - and that spooks me!