Please note, that this will only work for errors that get logged into the DNN Event Log. Also, you will need query access to your database.
All you have to do is run this simple query, and your last error that the site experienced will load in your query window:
SELECT TOP 1 CAST([LogProperties] AS XML)
ORDER BY [LogCreateDate] DESC;
If you are using a version prior to SQL Server 2005, you can still perform this query, although you are going to have to adapt it to XML yourself (in regards to the line breaks and formatting). Here is your version of the query:
SELECT TOP 1 [LogProperties]
ORDER BY [LogCreateDate] DESC