Bit of an odd one.
I have a recent project I have been working on. (in Cubase 9.5) on mac
Loaded it up today and am getting no sound from playback or from playing on my keyboard. It was all working fine last night.
All the meters are showing the sounds are playing.
The meters on my external mixer are showing that the sounds are playing.
But the sound is not coming out of the speakers.
Now, if this was an issue with the Scarlett 18i20, or control room, or studio settings, this would be understandable.
BUT… I can load ANY of my other projects and they all play fine.
I do not have any channels muted or any channels set to solo.
Anyone have any ideas?
Are the tracks set to “Record Enable” or “Monitor”? They both need to be disabled and maybe you missed that on this project.
Regards 
Doesn’t change anything if they are on or off. Still completely silent.
I didn’t change anything last night and it was all playing back fine when I saved it.
You didn’t give much detail for the type of tracks that have no sound. If “Instrument Tracks” are the VSTi (s) enabled? They could be “Bypassed” and look like they are enabled.
Regards
Silence is from everything.
That is VSTi and sampler tracks. All silent.
All meters on main panel and control room panel are showing the sounds are playing.
The meters on my X-Touch mixer all show the sounds are playing. But no sound comes out of the speakers for anything at all.
If I add a new VST or even directly drop an audio sample into the project, there is no sound from these either.
But if I load any previous projects (except the one from last night), everything plays back fine. All sounds are working as they should.
Came up with a “workaround” rather than a solution.
Decided to open a new project, then drag and drop everything from the project that had no sound into the new one and go from there.
Sorry I had to leave but I am glad you found a workaround. 
Regards