@Anonymous hi,
in AutoCAD Model tab (space) to have a drawing in another scale does not require scaling. instead you should plot it at the right scale. for example if your drawing is in mm and it is scaled to 1:50 and you want it in 1:100? you plot it at 1=100 plot ratio (of course there is need to fix the annotative objects [like texts, dimensions] to the new scale)
by doing a scale of 0.5 (for example) you took a measured line of 10000mm and made it 5000mm. that's means if you draw a new dimension on it you will get a measure of 5000 but it's not?!
what happened to you is after scale, although the drawing scale went down but the dimension global scale (known as global dimscale) was not cause dimensions are controlled by a dimension style and each scaled drawing need to have it own dimension style.
So to fix the state you entered, you need to scale the drawing back up and correct the global dimscale for the current dimension style. but the better way for you it is to create a new dimension layer and copy the existing dimension to that new layer, create a new dimension style with the corrected global dimscale and apply the new dimension style to those copied dimensions and yes i know maybe some will jump in now and say there is other way to do it and it's call annotation scale so i say to them: the op is a beginner and before he use annotation scales he need to learn how to use dimension and dimension styles 😀
Moshe