no audio with exsiting project

I am having an issue that sometimes when I open a project I do not get any sound . when I create a new project I am able to get sound. it is occurring randomly. I have checked and correct device and outputs are there. nothing happens when I switch the audio device. I use a macbook and have updated to latest version of cubase artist 10.
can someone help me identify what may be the root cause of this? I have been struggling for quite some time now.