What is the best thing to do when a business isn't open during the posted hours of operation?

There is no legal action that you can take against a local business not adhering to their posted hours. If they are part of a larger corporation, you may report them to that corporation. However, this has been an unusual year for business due to COVID and the many restrictions associated with it. This could be the reason.

Well, it's winter. And then there's COVID.  They may not be savvy enough to make up one of those clinging appliques to stick on the door stating hours during COVID are limited.

My guess is that they have at least a phone. Call the number and ask, or leave a message for them to inform you of their updated hours of operation.

I call  stores all the time. I've wandered the Big Box Stores for the last time, after driving to them and finding that in spite of the information on their web site, they didn't have product X or Y in stock. Maybe they did have it, but it wasn't in Aisle 11 Bay 23 like the computer said, and no employee was motivated to go look for it. Buy online, pick up in store. 

I've known a couple stores that have followed the M.O. you described. In both cases, the owners were trying to keep the business afloat while caring for elderly relatives or resolving their own health crisis.

Maybe they're not operating at all? Due to COVID or other reasons. Do they have a website or a phone number to get in touch to find out what your options are?

Awwww----poor you. You can't go to the jewelry store. Seems like a first-world problem to me. 

I can't think of a single reason why you would need to "report" anyone. It's none of your business why a store would be closed. Obviously there are problems, and you know nothing about those. So butt out and drive the 30 miles if you absolutely MUST go to a jewelry store. 

Special kind of stupid to not realise covid closed multiple businesses in the past year. 

You call ahead.  Jewelry stores are often run by families or a single individual.  The Chamber of Commerce would politely listen to you and then have a good laugh at you when they hung up the phone.  

Because of the pandemic and changes every week, some stores are not sticking to normal hours.  They should have a sign posted that says "please call for hours" or something like that.  Because they don't have a sign, it is a possibility that they might be closing permanently. Call them and if they answer, then they're probably open.  Let them know you've been going there but it's been closed, and ask which day they'll be open for sure so you can plan your visit.

Covid ??? Perhaps they are ill. These are private premises they can open when they like.

Before you go, call in advance and ask whether they are open.

The business is free to operate however they want.

The Chamber of Commerce doesn't care.

The BBB doesn't care.

If you don't like it, you drive the 15 miles to the other store... you are the one that decided to live in the middle of nowhere.

Or you could call and make an appointment

Or try to catch them when they are in. 

But the business is free to not open when they don't want to.