That would be my guess Marcia. I should have said something when I saw an email to you bounce (Friday night?), but we get alot of that here and it is usually just a one time thing.
Tracing everything out to your name servers appears they are up and running. If you had any changes done recently, I'd guess they didn't get them right on the server end. This is one where you'll have to get with them to get it fixed.
If you can still access other sites on the same server, that would suggest a problem with the DNS record for your site specifically, rather than a problem with the whole NS.
I had a wierd problem recently with a client site where a breakdown in communications led to the company we use for secondary DNS provision still having a primary zone file, which of course conflicted with our own. This led to the client site being unavailable (giving a DNS error like the one you describe), but only sometimes, dependent on whether the route you were using in that browser session led to the "right" primary NS or not, all because there had been a fault in DNS replication.
Its quite rare, but can be hard to spot, because everything looks right. I dont know if you are using a similar set-up, but it might be worth bearing in mind