Is it possible that somebody has cleared or acknowledged the latest alert?
The only way I can think of retrieving the data is to look at the SQL table 'AlertLog'. Do this:
From your Solarwinds server, open 'Database Manager'.
Click 'Add default server' and then browse to the correct database (e.g. NetPerfMon). Expand this and look for 'AlertLog'.
Execute a query against this (e.g. SELECT TOP 1000 * FROM [dbo].[AlertLog]) and look at the returned results to find the alert that triggered.
If this doesn't help, then perhaps raise a support case with Solarwinds?