Do they have a phone number on their website? Have you tried calling them?
As someone else said, small business are often closed on weekends and if they have been busy, it might take them some time to get caught up on their e-mail. And of course, it is always possible that they could be having computer or e-mail issues.
We have had it happen that we answer a customer's e-mails - even multiple times - and our answers have not gone through. Spam filters, greylisting... any number of things can happen that are nobody's fault. Please give them some time and, when in doubt:
Don't be afraid to use the telephone. Even if it is old-fashioned.