Trouble with Microsoft Outlook 2007

I use Microsoft Outlook 2007 and, although it opens ok, I get the following error message when I click "Send/Receive":

"The server name you entered cannot be found on the network. It might be down temporarily. Verify that you are online and that the server name is correct.

My technical knowledge isn't that great but I have checked the settings in the "Accounts" tab on Outlook and everything seems to be as it should be.

Any ideas anyone? This isn't the time of year to lose email!!

Many thanks.