Reset Permalinks to original

I have installed wp as my website in the root directory. The blog was running well. However few minutes ago i tried to change the permalinks from default to Date and Name based. then i tried to update the permalink structure and then i found out that the .htaccess file were not writable. now the permalink structure is changed but it’s not working well in the blog.

Now i would like to change the Permalink structure to Default, but the permalinks page under the Options in the WP Admin can’t be accessed. i tried to open in the browser, it says page can’t be displayed.

Can anyone help me how to solve this problem and how to reset the permalink structure to ‘default’?

Thanks in advance