My HDRW 720 has not been able to access the TV Guide channel for several weeks. I have tried unpowering and going through the setup several times. The cable wiring has not changed and I have checked that the connections are good.

Any ideas on what could be my problem?

Have you checked that the Host channel for the Guideplus epg hasn't been turned off,in the Channel Editor menu.(Is it ticked?)
Change the host channel from Automatic setting to Manual setting,and then manually set Itv1,asuming you have a Terrestial aerial connection or Eurosport,if you only have a Sky digibox or other satellite receiver connected.

The HDRW720 doesn't have the manual/auto setting in the menus like the later models. You can set the host channel by leaving the box (powered up) on the host channel until it downloads the EPG, then make sure the Guideplus EPG is on screen before putting the machine into standby.

I would do the checks TV D suggests first. I would also suggest a complete system reset - hold the standby button in whilst connecting the power - followed by a new set-up.

Failing that, if you are using Eurosport from a Sky box as the host channel, tried rebooting the Sky box by unplugging & reconnecting. There is a known issue with some Sky boxes which occasionally prevents the reception of Eurosport teletext, and hence the Guideplus data.

Bear in mind that it will take up 24hrs to populate the EPG after any of these actions.