wooocmmerce login page working one day

I am working one woocommerce project,Login page working one day,The next day one value is changing..

function loginuser(){ ?>
    <form method="post" class="login">


            <p class="form-row form-row-wide">
                <label for="username">Username or email address <span class="required">*</span></label>
                <input class="input-text" name="username" id="username" value="" type="text">
            </p>
            <p class="form-row form-row-wide">
                <label for="password">Password <span class="required">*</span></label>
                <input class="input-text" name="password" id="password" type="password">
            </p>


            <p class="form-row">
                <input id="_wpnonce" name="_wpnonce" value="8e4df" type="hidden"><input name="_wp_http_referer" value="/thanks-for-registering/" type="hidden">             <input class="button" name="login" value="Login" type="submit">
                <label for="rememberme" class="inline">
                    <input name="rememberme" id="rememberme" value="forever" type="checkbox"> Remember me</label>
            </p>
            <p class="lost_password">
                <a href="http://ift.tt/19m1woW">Lost your password?</a>
            </p>


        </form>
<?php }

and this value(value="8e4df") is change every day,how do i get <input id="_wpnonce" name="_wpnonce" value="8e4df" type="hidden"> in fonctions.php can anyone help me...

I tried value="<?php wp_create_nonce( $action );?>" but it is not working...