Strange sound loss

I have encountered something odd.
I have been going to and from a project for a couple of monhs and its run very smoothly.
Only yesterday I returned to it and the project loads, all th stems are shown.
If I press play I hear nothing, I can see the sound moving up and down but hear nothing.
I have checked my driver and the routing, and i all looks normal.
Sometimes, if I leave it before playing it will all of a sudden kick n and I can continue.

This is inconsistent and can take a long time.
Every track is midi at the moment and Im ru ning 5 or 6 Kontakt instruments.
The effecs, such as reverb or delay, are all running throught the vst pug in rather than an external plug in
I know that in the past I have had lag problems because I wasn’t sending reverb effects before.
Maybe this has something to do with it.

Any wise words?

Im using cubase elements 9 by the way