If you haven't changed channels on the main box or switched it off then it should be able to rewind for around 3 hours. If it can't then it's a possible hard drive problem. You could try a factory reset to see if that helps.
To factory reset go to Settings (the cog at the end of the Home menu) > System > Factory Reset. It's actually misnamed and only a partial system reset and not a complete factory reset. You are given 2 options
Keep Recordings
Format Disk
Choose Keep Recordings, that means you won't lose your recordings and any planned recordings or series links you've set up. You will have to sign into your apps such as iPlayer, Prime, Netflix and ITVX again. Also Match frame rate and Standby power consumption might go back to their default settings if you have changed them.