Viewer freeze

I have a project that’s been fine until tonight. The project timeline plays fine, but if I try and play footage in the viewer, it just plays 1 frame and stops? I have re-started the Mac as well as the software. Been working for the last week absolutely fine, then this. No updates today, or changes to anything else. Any ideas? Thanks Damian 

Two things you can try in the order given .
1. Select your library, and then from the top menu in FCP select FILE - delete generated library files. From the dialogue that appears, choose to delete ALL render files. Give it a few seconds and then try to playback the timeline to see if it now works properly. If not, try below.
2. Try trashing your FCP preferences by launching FCP while holding down OPTION plus COMMAND keys. Select to delete preferences when prompted. When FCP starts back up, locate and load your library, open your project and see if it now works.  NOTE- when you trash FCP preferences you will need to set them again in the app.  By default "Background Render" will be turned on.  Turn that setting OFF (which will require you to manually render any timelines if you need to, but this is better than having FCP automatically do it which can cause corrupt render files on occasion).