This article is for Timesheet Reports and Gadgets for Jira Server. For Jira Cloud or Time Reports for Jira Cloud or Server there is Log Work button and In Progress Issues in Timesheet and Time Entry.
There is no 'Log Work' button at the bottom of the timesheet gadget or report in Timesheet Reports and Gadgets for Jira Server version of the plugin, only in the Cloud version.
However using Time Sheet Summary gadget in single user mode, currently logged in user can still log worked hours from dashboard. Time can be entered for previously worked issues, or any issues matching filter configured in Administration - Add-ons - Timesheet Configuration with In Progress issues option.