Try deactivating all plugins (yes, all) to see if this resolves the problem. If this works, re-activate the plugins one by one until you find the problematic plugin(s). If you can't get into your admin dashboard, try resetting the plugins folder by FTP or PhpMyAdmin (read http://bit.ly/UhpkLl if you need help). Sometimes, an apparently inactive plugin can still cause problems. Also remember to deactivate any plugins in the mu-plugins folder if you have one. The easiest way is to rename that folder to mu-plugins-old (read http://bit.ly/WMrMM5 for more information on mu-plugins)
Try switching to the Twenty Eleven theme to rule out any theme-specific problems. If you can't log in to change themes, you can remove the theme folders via FTP so the only one is twentyeleven. That will force your site to use it.