it said the site revocation certificate wasn't available
It's very unlikely it was actually revoked, that rarely happens. You said the error was that the revocation information
wasn't available, it doesn't mean it was revoked, only that your computer couldn't determine if the certificate had been revoked or not.
There are a few reasons why your computer may not be able to confirm if the certificate was revoked or not, like Jasmine said it could be the date on your computer so definitely check that, but it could have just been a temporary connection problem with the certificate's revocation server (used to check validity). Unfortunately that error can have lots of causes, it can even display when certificates are nearing expiry even when they're still valid. You can also check that you have all the latest Windows updates (including updates from the
Optional section of Windows Update) because some of the updates are for certificate checking.
If the problem still exists, can you please post the first bit of the URL that is giving the error message (don't give the full URL since you're visiting the cashier so the URL may contain some account information, just give the address up to the first slash
/ ), then we can check if the problem is with their certificate or just with your computer.