There can be 2 case.. either your system restore is turned off or you system restore files are corrupted..
just check out whether it is turned off or not.. rigth click my computer -> go to properties -> click system restore tab..
Here it shows that system restore is turned off or on for which drives....
If its off than switch it on and than you can create a new restore point.. however you wont get the previous one back..
Now if the files are corrupted than go to the same location and switch the system restore off.. restart you computer... all the restore files will be deleted... than again switch the system restore on and create a new restore point....