This happened to me once, it was something related to my browser, so reloading my PC worked.

Other things you could do:

(1) Open the browser console to see which resources are taking time or not loading

(2) reload static files & compress styles with python manage at the openstack_dashboard directory (in ubuntu: /usr/share/openstack-dashboard/openstack_dashboard/)

python manage.py collectstatic --noinput
python manage.py compress --force

(3) check if any process is taking more CPU/Mem resources at the controller's host.