Why iMac showed a problem at boot time?

Hi everybody

Since a week, more or less, my imac seemed to be slower than usual at power up/down time (particularly after Mountain Lion upgrade), but operativity was normal and my most used apps didn't show any problem.

Few minutes ago at power on a gray screen came up and a requester suggested to me, among other things, to reinstall mountain lion, to recover from a time machine back up. Apparently loss of data seemed to be the only possibility, but I decided to reboot, and, by the chance, OS came up. From disk utility checked both Macintosh HD and Bootcamp (Windows 7) and also the USB HD's usually connected to the mac and all were ok.

Have any idea of what could have been happened or suggestions of how to better investigate?

Of course I did immediatly run time machine that was a bit neglected lately in order to have a fresh backup just in case.

Thanks a lot, Gianfranco