I have lot of story in my mind but whenver try to write, my mind become blank and cannot collet all thoughts to write story.
The problem you are experiencing is probably that you have too much in your mind and when you try to write, you don't know where to start and in what order to write it all down, and you are afraid that you will mess your wonderful idea up.
There are two things you can do:
Chose one scene or situation and begin writing from there. Forget about order and all the things you think you need to get into your story, and just write from that scene forward, discovering your story as you go along.
That starting scene doesn't have to be the beginning of your story, you can write out of order and write parts that come before that starting scene later.
Also, do not worry that you will forget something. It's all in your mind, and you will remember to put it in when the appropriate place in your story comes. Or you can add things in later, when you revise your story.
Write down all your ideas that are related to this story on index cards (or into a text document). Do not worry about how they all should go into the story, just put them on paper (or in a text file), so you have them before you.
Now order them. Take each of the index cards, think where in the story this should go, and put it in a line in front of you, with the first parts of your story to the left and the ending to the right. (If you have a text file, copy and paste the notes to where you want them to be in your document.) Number the index cards, so you can easily put them in the correct order again, if you accidentally mix them.
When you have ordered your ideas into an outline in this way, open a (new) text document, take the first index card (or the first note in your other text document), and start writing from this beginning to the next index card (or note). Fill in, what happens inbetween.
When you write, using either of the methods briefly sketched above, you can write in several stages. Write a rough draft of your story first, and then flesh it out, adding more and more detail with each revision. This takes away some of the pressure of writing a publishable, "finished" version at the first attempt.
Just think of the first draft as "finding" the story. It doesn't have to be perfectly written, there can be inconsistencies and parts that you just write down keywords for, to flesh out later. Polishing the prose can come last.
The only think I can suggest is to sit in front of your computer at a blank page and write. If you can't think of things to write, then just write random words.
Decide on a given number of words to write each day. I would start off at 1,000 if you're not used to writing. This may sound daunting, but it is very doable if you can type at all.
This exercise is designed to get you into the habit of writing. At first, what you write will be random and have little or nothing to do with your story. Maybe it will just be the main character's first name written 1,000 times. It doesn't matter.
The theory behind this is that there is a creative part to your mind that wants to express itself, as well as a critical part that doesn't want you to write at all. By writing randomly you are bypassing the critical part and letting the creative part have free reign. Eventually the creative part learns how to express itself bypassing your internal critic.
Eventually, you will learn to write with both the creator and the critic working together, but that is a long time away.