I’m completely unable to reproduce this behavior. I tried a few different sample images, with/without adding selections to the ROI manager (ctrl + t), and with/without various measurements checked. A length measurement was always displayed and looked reasonable (except when I measured with no selection, which produced measurements for the complete image).
Can you confirm this behavior with any of the sample images? (File>Open Samples menu)
If so, is your ImageJ/Fiji up-to-date?
Can you provide information about your operating system and java version?
Even a screenshot showing the length=0 measurement would be helpful.
I reckon I had a similar problem some times ago, that was simply due to a wrong scale (although I don’t remember how it was even possible). Try opening the image properties and set an arbitrary unit, pixel width and height.
I found out myself that you can select the proper tool from the tool menu by right clicking the measure stick icon (this gives you a drop down menu of different measuring tools). This way you can use the freehand tool. After calibration you can measure curved distances with this tool.
Glad that it works for you. I guess you mean the Freehand Line tool, which is different from the Freehand (area selection) tool. Have a look at the user guide section on line selections for further reference.