Gulp suddenly stopped watching changes, compiling scss to css and opening and reloading page

var gulp = require("gulp");
var sass = require("gulp-sass");
var watch = require("gulp-watch");
var csso = require("gulp-csso");
var concat = require("gulp-concat");
var browserSync = require("browser-sync").create();

/* Static Server + watching scss/html files */
gulp.task("serve", ["sass"], function() {

    browserSync.init({
        server: "./"
    });

    gulp.watch("*.html").on("change", browserSync.reload);
    gulp.watch("sass/*.scss", ["sass"]).on("change", browserSync.reload);
});

/* Compile Sass into CSS & auto-inject into browsers */
gulp.task("sass", function() {
    return gulp.src("sass/*.scss")
        .pipe(sass().on('error', sass.logError))
        .pipe(gulp.dest("css"))
        .pipe(browserSync.stream());
});

/* Concatenate and minify CSS */
gulp.task("css:build", function() {
	gulp.watch("css/*.css").on("change", function() {
		return gulp.src("css/*.css")
		.pipe(concat("style.concat.css"))
		.pipe(csso({
			comments: false
		}))
		.pipe(gulp.dest("css_result"));
	});
});

gulp.task("default", ["serve", "css:build"]);