My 4 year old cairn terrier hasn't eaten in 5 days! Please help!?

She's been drinking water but refuses to eat anything, even her favorite treats. Sometimes she wont eat her dog food For a couple of Days, which is normal for her but after the third day we knew something was wrong. We want to take her to a vet but she doesn't have any tags. Would that cause a problem? If you have any advice it would be much appreciated! I'm very worried about her! :(