US embassies are usually well guarded. Each embassy makes up their own rules of access. If you have an emergency you should be able to get ahold of someone in the embassy (either in person or by phone). However, embassies aren't airports and aren't open 24/7 for you to mingle around in. So save for an emergency, they do have business hours. Some do things only by appointment, while others may have walk-in hours. Check the specific embassy's website as to their policy.