Have you checked the event logs? There should be an entry for any BSODs and minidump creations in the System events.
[Edit: Checking event logs]
In the event viewer, you're looking for Information
entries from Save Dump. Open View -> Filter and select
Save Dump in the source drop-down list. (You could also filter on EventID 1001.) Hit OK.
This should leave you with a list of entries sent by the "Save Dump" process
during a post-BSOD bootup. Double-click to show the full entry -- the
description gives the location of the saved dump.
If there are entries but the files don't exist, your dumps are gone.
Maybe they were deleted by a system cleaner tool (CCleaner or the like).
If there aren't any entries, no dumps were saved.
Make sure you've checked "Write an event to the system log" and
selected "Small memory dump" in the Startup and Recovery dialog.
You can also change the save location to whatever you like (eg, "C:\dumps").
Original: I'd try using BlueScreenView; it may find existing minidumps automatically. If they don't exist it'll be helpful once you get them going again. Freeware, works under 32- and 64-bit Windows XP/Vista/7/etc.