One major source of frustration with being an author is feeling like you want to write, but you don't have any inspiration. Specifically for fantasy writers, it's difficult to start a new story if you don't have a magical world or a new set of characters to experiment with. I know I spent many hours during my childhood wracking my brain for a new concept for a book, and after writing for a few years I've come up with my own method to get my creative juices flowing.
I eat a lot of sugary cereal, then watch anime at midnight.
I do not recommend this method.
Let's look at some healthier alternatives!
Take a cliche and subvert it
A cliche or a trope is a well established plotline in a specific genre, something used over and over. An example would be having a "Chosen One" as the main character, dead parents, or a royal killing his brother to seize the throne. Most people are told to avoid cliches, but I disagree. They aren't a bad thing. The reason these ideas have become overused is because they work. Something that I like doing is taking a cliche and writing my own version of it, with a twist. What if there was a kingdom where nobody actually wanted to be king? What if someone was burned at the stake because they didn't have magic? What if the Chosen One rejected their destiny? (A great example of this would be Avatar the Last Airbender.) Play around with it.
Pay Attention to "What If..." and "I Wish..." statements
Design your own world. What do you want to be different? What do you wish? A great creative exercise is to question why things are the way they are. What if humans had tentacles instead of hands? Would that be weird, or normal if that's just the way we were? This makes your brain shift away from linear thinking and allows you to be more creative. This also allows you to come up with new magic systems and races of creatures. Do you wish you could read people's minds? Make an alien race that can read minds.
Who are your invisible friends?
Personally, there are two ways to come up with a new book idea. You come up with the worldbuilding and concept first, then assign it a main character that would best fit that environment, or the other way around. Sometimes you come up with an idea for a character before the world. After all, the characters are the most important part of the story. Did you have any invisible friends growing up? What were they like? How did they act? Did they have any magic powers? You could make them the main character of a new book. Another idea is to take one of your own character flaws and design someone with that same flaw. What do you struggle with and wish you were better at? Humility, intelligence, standing up for yourself? Make that something a character has to deal with and eventually overcome.