These moments occur with everyone but the more experienced performer can cover their tracks with a bit of "intimate chat" or "audience participation" and turn their blank moments to advantage while mentally regrouping. Playing in a band, as I do,it's not nearly as bad a problem and we've played together enough to keep going without many (if any) of the audience spotting one of us going to bits! We've done some llllooonnnggg versions of songs while the singer was waiting for a prompt to take us out of the song! People have actually commented on how much they liked "the new arrangement"