Write a troubleshooting section in user guide

docs/user_guide/source/troubleshooting.rst

 Troubleshooting
-===============
+===============
\ No newline at end of file
+
+Agkyra hanged up expectingly
+----------------------------
+
+Maybe it's just the GUI.
+
+The GUI is a separate component (it is even developed in a different
+programming language to the rest of the application). Use the command line to
+check if the application is still running:
+
+.. code-block: console
+
+    $ agkyra-cli status
+
+If the status is ``Syncing``, ``Pausing`` or ``Paused``, then the application
+is running. Try running agkyra again and see if everything is OK.
+
+Otherwise, see the next issue.
+
+Agkyra is not starting
+----------------------
+
+The application has to be reset manually. We can do this without data loss.
+You need to remove the session locks to enable the application to start again.
+To do that, remove the files ``$agkyra_dir/session.db`` and
+``agkyra_dir/session.info``.
+
+.. note:: To check the exact value of ``agkyra_dir``
+
+    .. code-block:: console
+
+        $ agkyra-cli config list global agkyra_dir
+
+Agkyra is still not starting
+----------------------------
+
+Probably the helper (scripts/server.py) is in stale. Find the process and kill
+it, e.g.:
+
+.. code-block:: console
+
+    --- in unix-like enviroments ---
+    $ ps aux|grep server.py
+    user 1234 ... ... python /home/user/agkyra/scripts/server.py
+    $ kill -9 1234
+
+I still have problems
+---------------------
+
+Please, contact us at TODO