This seems to be a recurring question on this forum, but I haven't seen an answer (other than rebooting) that solves the problem.
In my case, the wingpanel shows for a second, looks like it is trying hard to push down from the top of the screen, then, after a few attempts dies a sad and lonely death.
Here is what terminal reports when attempting to run wingpanel:
$ wingpanel
(wingpanel:5077): Gtk-WARNING **: Theme parsing error: gtk-widgets.css:2045:14: 'min-height' is not a valid property name
(wingpanel:5077): Gtk-WARNING **: Theme parsing error: gtk-widgets.css:2046:13: 'min-width' is not a valid property name
(wingpanel:5077): Gtk-WARNING **: Theme parsing error: gtk-widgets.css:2051:13: 'min-width' is not a valid property name
[INFO 19:43:29.254521] Application.vala:153: System Panel version: 2.0.2
[INFO 19:43:29.254562] Application.vala:155: Kernel version: 4.4.0-77-generic
[FATAL 19:43:29.309470] display_widget_set_icon_name: assertion 'self != NULL' failed
[FATAL 19:43:29.309798] display_widget_set_icon_name: assertion 'self != NULL' failed
Error: GDBus.Error:org.freedesktop.login1.NoSuchUser: No user 1002 known or logged in
Error: GDBus.Error:org.freedesktop.login1.NoSuchUser: No user 1002 known or logged in
[02:43:29.435759 Gtk-Warning] Theme parsing error: gtk-widgets.css:2045:14: 'min-height' is not a valid property name
[02:43:29.435789 Gtk-Warning] Theme parsing error: gtk-widgets.css:2046:13: 'min-width' is not a valid property name
[02:43:29.435816 Gtk-Warning] Theme parsing error: gtk-widgets.css:2051:13: 'min-width' is not a valid property name
[02:43:29.462108 Critical] string_to_string: assertion 'self != NULL' failed
[02:43:29.507269 GLib-Critical] g_hash_table_get_keys: assertion 'hash_table != NULL' failed
[02:43:29.534970 Gtk-Critical] gtk_window_resize: assertion 'width > 0' failed
[02:43:29.650466 Warning] [DesktopFileInfo] Not Application-type desktop entry
[02:43:29.652287 Warning] [DesktopFileInfo] Key file contains key 'Hidden' which has a value that cannot be interpreted.
[02:43:29.698815 Warning] [DesktopFileInfo] Unable to get exec for Other
[02:43:29.752826 GLib-GObject-Warning] cannot register existing type 'BluetoothServicesObjectManager'
[02:43:29.752840 GLib-Critical] g_once_init_leave: assertion 'result != 0' failed
[02:43:29.752848 GLib-GObject-Critical] g_object_new: assertion 'G_TYPE_IS_OBJECT (object_type)' failed
[02:43:29.752855 GLib-GObject-Critical] g_object_bind_property_full: assertion 'G_IS_OBJECT (source)' failed
[02:43:29.787003 Info] Tweaks.vala:307: Activating Tweak plug
[ERROR 19:43:29.854136] [GLib-GIO] Settings schema 'org.pantheon.terminal.settings' does not contain a key named 'opacity'
Trace/breakpoint trap (core dumped)
Is there a fix to this missing wingpanel issue? Without wingpanel, I can't really use the OS.