Cisco Router Save Configuration

I have a cisco 2501 router that refuses to keep a configuration after it has been powered down. This has been going on for quite some time but since I generally only have to power down the router once a year, I have just simple reloaded the configuration.

It seems to have started after we had to perform a procedure on the router to correct the problem of a forgotten password. However I can no longer remember the procedure that was performed. (NOTE: This may be just a coincidence.)

Any ideas?