You will usually see this error because your computer cannot connect to the SQL server for your organisation.
You will need to refer this to your IT department for assistance, however, to help them, some areas to check are:
Is it only you seeing the error, or are your colleagues seeing the same error?
If all users are seeing this then it is likely that the problem lies with the SQL server.
If only you are seeing the error, check the ODBC connection on your machine to ensure that this has the correct details. This article explains how to setup the ODBC connection correctly.
When referring this to your IT department, it can be helpful to mention that the error you are seeing in Dimensions is a symptom of an issue between your computer and your server. This can be caused by a range of issues including:
Network issues
Domain Name Server (DNS) issues
Issues with the services on the SQL server
If an alternative server is available you could backup the database, restore it to the alternate server and setup Dimensions to connect to that. It is not ideal, especially if the other server is not a full-featured or as powerful, but if the original server is proving uncooperative this may allow users back into Dimensions until the original server can be restored to normal operation.