For me, I think if that sort of blanking mentioned by the OP is going to happen, it's going to happen and there's nothing you can do about it. If one is singing regularly, one might become more adept at recovering from the situation but sometimes there is no way back.

I can try to run through words in my head before a song to see if I remember the words (if I ever knew them all the way through to start off with) but I don't do it as often as I perhaps should and it's not always that easy to do.

The place I'm most likely to sing in is a mixed session. I'm joining in a lot of the time, whether I start a tune or song may depend on what instrument I have in my hand after the person before me (usually goes in a circle), the way I feel the "mood" of the room is going... it's pretty spontaneous. It's not long back that for some reason I decided "The Last Farewell" (made popular by Roger Whittaker) would fit as something a bit different and I've only ever known the 1st verse of that.