I am using Frenzy1 (based on FreeBSD6.1) installed on the hard disk, every so and so the system crashes, mainly when I play with it tweaking it, etc... I don't mind the crash but then when it reboots I can not get FreeBSD to start again, it freezes rebooting in the middle and I get the message:
/ was not properly dismounted
R/W mount of / denied. Filesystem is not clean

And thats it, the only way around it is reinstalling.

Any clue about what I can do or what provoques that message so I may not do it again?

Notice that I have no command prompt because the computer freezes in the middle of the boot and does not give me a change to use the command for anyting, I tried booting in single and safe mode but the same thing happens.