I am running Ubuntu 18.04 with the XFCE interface and QGIS v3.6. When I start QGIS the menu bar at the top of the window is missing, and I can't find any way to bring it back. I found some solutions online that suggest modifying the QGIS2.conf file, but I don't have one of those.
Do any of you have ideas how to solve this? The main reason I use QGIS is to save quick previews of images, which is no longer possible without the menu bar!
(edit) Rostranimin suggests launching from the command line with the no configuration option, I had actually already tried this (but neglected to mention it). Doing so still results in no menu bar. (edit2) To give more background info: I was previously using Ubuntu 14.04 on this machine and whatever the default version of QGIS was for that distro (2.4?), then my company upgraded me to 18.04 with QGIS 3.6. In 2.4 the menubar was visible, it hasn't been visible since the update. Unfortunately as this is a company-managed machine I don't have sudo access, so my options to change things are limited.
I have another machine (self-managed) that's running Mint 19 with QGIS 3.6, the menu bar appears fine there.