Have you tried:
- deactivating ALL plugins temporarily to see if this resolves the problem. If this works, re-activate them individually (one-by-one) to find the problematic plugin(s). If you can't get into your admin dashboard, try deactivating via FTP or SFTP or whatever file management application your host provides. If applicable, also remember to deactivate any plugins in the mu-plugins folder. The easiest way is to rename that folder to mu-plugins-old.
- To rule out any theme-specific issue, try switching to the unedited default, core-bundled Theme (such as Twenty Twelve or Twenty Thirteen) for a moment using the WP dashboard. If you don't have access to your admin area, you can switch to the default theme by renaming your current theme's folder in wp-content/themes and adding "-old" to the end of the folder name using via FTP or SFTP or whatever file management application your host provides. Alternately, you can remove other themes except the default theme. That will force your site to use it.