Lighting suddenly won't build

I have a large project I’ve been working on for a few months and no matter how large and complex the scene got, I could always build lighting just fine - until now.

Now whenever I try to, literally nothing happens except I have the asterisk by the project name telling me I can now re-save it fresh. Whenever I try and ‘build all’ it does everything except the lighting - under no circumstances does it even bring up the ‘building lighting’ percentage window on the bottom right corner of the screen regardless of what I try and build. I had 1 error pop up once telling me I needed to rebuild the lighting but that’s just it - I CAN’T.

I’ve seen that emptying the swarm cache might help but at no point have I managed to find out how I can actually do that to begin with.

Any other solutions? This is literally the last thing before it’s completely finished and I really just want to get this done and out the way for the last few weeks.

Much obliged.