Your Jenkins pipeline jobs will not run. You see "Build Scheduled" when you click on the "Build now" link. But nothing happens. You see an error that your /var/lib/jenkins/ is running low on disk space. You have no access to the back-end of the server that runs Jenkins. What should you do?
Look at the dashboard view. Next to the date and time of the most recent success of the job (build, project or pipeline) will be an integer number (e.g., 1, 53, 840 etc.). Find a job that has the largest integer value. See if you can delete the previous builds -- or more efficiently, delete the entire project. It may take 30 minutes, but eventually this will free up disk space.