I know this post is old but this could help someone.
You might have removed the style.css file, it is located in the root of your theme (wp-content/themes/your-theme-name/style.css).
A way of knowing if this is your problem is going inside "Apparence>Themes" and looking for this error: "Error: Stylesheet is missing".
If you do have the error you can copy this and put it inside your style.css
/*
Theme Name: your-theme
Theme URI: https://wordpress.org/themes/twentytwelve/
Author: the WordPress team
Author URI: https://wordpress.org/
Description: The 2012 theme for WordPress is a fully responsive theme that looks great on any device. Features include a front page template with its own widgets, an optional display font, styling for post formats on both index and single views, and an optional no-sidebar page template. Make it yours with a custom menu, header image, and background.
Version: 1.0
Requires at least: 3.5
Requires PHP: 5.2.4
License: GNU General Public License v2 or later
License URI: http://www.gnu.org/licenses/gpl-2.0.html
Tags: blog, one-column, two-columns, right-sidebar, custom-background, custom-header, custom-menu, editor-style, featured-images, flexible-header, footer-widgets, full-width-template, microformats, post-formats, rtl-language-support, sticky-post, theme-options, translation-ready
Text Domain: twentytwelve
This theme, like WordPress, is licensed under the GPL.
Use it to make something cool, have fun, and share what you've learned with others.
*/
/* =Notes
--------------------------------------------------------------
This stylesheet uses rem values with a pixel fallback. The rem
values (and line heights) are calculated using two variables:
$rembase: 14;
$line-height: 24;
---------- Examples
* Use a pixel value with a rem fallback for font-size, padding, margins, etc.
padding: 5px 0;
padding: 0.357142857rem 0; (5 / $rembase)
* Set a font-size and then set a line-height based on the font-size
font-size: 16px
font-size: 1.142857143rem; (16 / $rembase)
line-height: 1.5; ($line-height / 16)
---------- Vertical spacing
Vertical spacing between most elements should use 24px or 48px
to maintain vertical rhythm:
.my-new-div {
margin: 24px 0;
margin: 1.714285714rem 0; ( 24 / $rembase )
}
---------- Further reading
http://snook.ca/archives/html_and_css/font-size-with-rem
http://blog.typekit.com/2011/11/09/type-study-sizing-the-legible-letter/
/* =Reset
-------------------------------------------------------------- */