The GUI shows a red message with the "Cannot retrieve license information from the Plastic SCM server. Retrying in XX seconds. Try again now" text.
Additionally, you can see a "No connection could be made because the target machine actively refused it 127.0.0.1:8087" error as well, but this depends on your workflow.
For some reason you local Plastic SCM server is stopped, you can start it from the Windows Services panel. Search for the "Plastic Server 6" service and start it.
If it doesn't start please send an email to support (support at codicesoftware dot com) explaining the situation and attaching the Plastic SCM Server debug file (C:\Program Files\PlasticSCM5\server\plastic.debug.log.txt)