Finding ideas can be hard when writing a story, but the reason why you can't think of one is because, you're thinking way over the limit. When finding an idea, you have to slowly think it over, thinking what should happen afterwards.

Write down any possible ideas you think of down, then read one by one, then improve it by writing a list of places and emotions that can relate to the idea. 

For example:

Sudden Kiss with Main Character - Beautiful, empty park, romantic.

List more than 3 improvements to the idea to give you a better image in your head. Remember, your ideas should best fit in your setting and story categories.

You should rarely think of one while writing, this will cause stress, because you're going to be forcing them out instead of letting them flow out. Write about 20 simple ideas, this can last for around 2 chapters. Look at your list and look at the improvements, and imagine them in your head. If you liked how it turned out, use it in your new chapter. If you don't, put a small cross on it, but don't erase it, you can use it next time when writing a new story.

Thinking of ideas may be hard, but it's there to guide you when writing, so keep the sheet that you wrote your ideas on, because you may need it in the future as a resource. 
Read a book, they may give you ideas. Read a newspaper, the news and your very own imagination can happen also, think about realistic occurrings to add more realistic features towards the story. Remember, you want your story to amaze your readers and let them be hooked onto it.

To calm you down, listen to some music while thinking, this will soothe and calm you down. Close your eyes, and imagine that you're in your character's position. Ask yourself; what would you do, and how would you react.

When you start to see something, let it continue, allow the idea to take over, when you open your eyes, write down what you saw, this could count as your new idea to be used.

Tip: Always/Often carry a small notebook with you. Decorate freely, scribble or doodle on it, just allow your ideas to run while. If you're like the arty person who loves to draw, sketch out a scene or design an OC. If you enjoy visualizing on things, write it down. The smallest ideas could be the greatest idea in your story. This little notebook is your gateway to an imaginary world - Your world. So do whatever you can in that book.
See something you like, WRITE it down!

Tip: If you never want to be in a writer's block, just dot down a little of your plot each day and finish it off whenever you're free!

Your imagination is what runs wild!