You are best backing up everything including the DB before doing any of this... before think about doing any of this.
If it is a plugin and you can't get into the WP backend, then you are going to have to brute force it a little bit. Login to the backend provided by your webhost, copy (download) everything in the plugin directory to your harddrive, then delete the plugins from wp-content/plugins. What should happen is that WP will deactivate the plugins since all the files are now missing. You could also delete plugins one at a time and try loading the site after each deletion.
The problem could conceivably be in the theme template itself. If it is, more or less the same procedure applies except that you want to leave the WP Default theme in place. WP should notice that the selected theme is missing and fall back on the Default.
Maybe, also, wait a little bit to see if anyone else has a better idea. I actually feel kinda bad about telling you do it this way. If, however, you really can't get in I'm not sure how else to deactivate the plugins.