when a old database was deleted ?

It could be an event that gets logged so you can check using the instructions on this page http://www.mssqltips.com/tip.asp?tip=1476 to query the logfile on your server.

If this isnt set up on your server then its unlikely you will be able to locate when it was deleted, other than restoring old versions of system databases to a test location and seeing when it appears in sys.databases. If you want to start logging database deletions then there is an article here (http://social.msdn.microsoft.com/Forums/en-US/sqldatabaseengine/thread/5b1b81c9-ff0f-49a0-8a8c-cd217954bfcb/) that explains what is necessary.