Dimension tool

For some inexplicable reason the dimension tool is not showing the actual number dimension. The line draws, and there is a space where the number should be, but there is no number. However, when I click on the middle of the line, there is something there, I have checked that it has not changed itself to white, the menu bar tells me it should be Calibri 24pt. But the actual 'dimensions' of the object is zero. If I change that to anything it changes resets itself to zero. On my other drawings all the actual dimension numbers have disappeared also. 

Any suggestions would be greatly appreciated as to what may have caused this. I haven't changed anything on the computer since I last used it.