Doesn't xp_readerrorlog take some parameters. If you don't specify any of the parameters, then it reads only the current log file.
Error logs can be cycled either manually or by retarting SQL Server. Typically, SQL Server retains backups of the previous six logs and gives the most recent log backup the extension .1, the second most recent the extension .2, and so on. The current error log has no extension.
So it could be that it is not in the current log file.
Just try opening the SQL Server Error Logs in SQL Server Management Studio.
1. In Object Explorer, expand a server, expand Management, and then expand SQL Server Logs.
2. Right-click a log and click View SQL Server Log.