Windows Vista Sleep has stopped working

Without me having knowingly changed any relevant settings, suddenly "Sleep"
no longer works on my Windows Vista.
Previously, when I selected Sleep from the switch-off options from the Start
menu, the system would shut down but when I re-started (by pressing the
physical "on" button on the computer), it would re-start quickly with all the
same programs and documents as before.
For some reason, this has stopped.
Does anyone know how to remedy this please?
Thnkas

The sleep mode is extremely unreliable and one of the many poorly
implemented features in Vista.
Microsoft is hyping Windows 7 as starting up so fast that sleep mode will be
irrelevant.
Alas WIn7 is but Vista SP3, another silk purse made from a sow's ear.
You are better off, as is the environment, shutting down your computer. I
doubt your time is so precious you can not afford to wait the extra minute
or so for the machine to reboot.