WordPress uses cookies for authentication. Clear your browser cookies and cache and make sure that your browser has cookies enabled. After doing that restart your browser and then try to log in. This should fix the issue right away. 

Switch off the plugins one by one by renaming them in your File Manager. Navigate to wp-content>plugins. After each rename reload the site.

It could also be caused by a coding error in the theme that you are using. You should try replacing your current theme with a default twenty ten theme. You can then select a new theme. Opt for themes that have recently been updated and has more than 100 000 downloads.

For more advanced users you can try to debug your site. Switch the debugging option on through your wp-config file.

 define( 'WP_DEBUG', true);