If you have installed a plugin which broke your installation so that the server doesn't come up anymore, you still can uninstall the offending plugin via the command line with pip:
pip uninstall <plugin>
Heads-up OctoPi users!!! For you it is ~/oprint/bin/pip uninstall <plugin>.
Note that <plugin> will usually be something like OctoPrint-<PluginName>, so for example if you wanted to uninstall the Autoselect plugin, you'd need to supply OctoPrint-Autoselect as the name to uninstall. If that doesn't work for you and you don't know the name, try pip freeze | grep OctoPrint (~/oprint/bin/pip on OctoPi!), it will show up in that list and you will hopefully be able to recognize it that way.