It depends on what you mean nothing to say.

Does it feel lik you have a wall in your mind and the words just won't come out? Or do you feel more like you have something to say but nervous to say it. Or, do you mean one second you're talking and the next was that your thoughts go blank and you forget?

I have a lot of thought blocking which is I forget what I'm talking about suddenly, even mid-scentance.

I get nervous and don't say anything about certain things, even though I know what to say.

Sometimes in my head is a wall and I can't get the words out I want to say no matter how hard I try.

I think the middle one is confidance, the top one possibly illness and the bottom one possibly illness, too... but hard to say exactly.