Its strange to understand the system restore got disabled on its own, normally it does not happen. I would first advice you to run a full anti spyware scan of you system to check if any malaware is doing this. Turning off System Protection for a disk deletes all restore points for that disk. You cannot restore the disk until you turn on System Protection again and a restore point is created. To enable system restore, there are few simple steps, since you have not mentioned the windows version you have, I am guessing it to be Vista. Here are the steps:
1. Open System by clicking the Start button , clicking Control Panel, clicking System and Maintenance, and then clicking System.
2. In the left pane, click System Protection. If you are prompted for an administrator password or confirmation, type the password or provide confirmation.
3. To turn on System Protection for a hard disk, select the check box next to the disk, and then click OK. – or – To turn off System Protection for another hard disk, clear the check box next to the disk, and then click OK.
Here’s to Good Karma