We are closed. This is a complete sentence in present tense for a store which has closed for the day. We closed... usually precedes an explanation... We closed the store for the weekend. It may be said as a response to a question. Why aren't you working this weekend? We closed... this is short for: We closed for the weekend.

It depends on whether you are talking about the situation NOW or an event in the PAST. Examples: NOW: You can’t come in now. We are closed. PAST: You can’t come in now. We closed 20 minutes ago.