I am running Windows Vista Home Ed. on a Toshiba Satellite. I have rencently started having problems with it going into sleep mode or hybernation without warning... it happens all of a sudden right in the middle of browsing a webpage or using a program. I can't get it to open up again and have to manually shutdown the computer.

you are not accidentally hitting the little button that gets hit when closing the lid are you (satellites are laptops... right?)? also, if it is a laptop, do you have it on battery when this happens or is it plugged in?

Yes... it is a laptop and I'm pretty sure it is not because I hit the button on the lid. It happens both on battery and when plugged in... it seems that it happens more when plugged in because at home I keep it plugged in most of the time. It happens while browsing the Internet without any warning... many times as I click on a link or start a new search. I'm not sure if it is hybernating or what... it does not shut completely down because the "power on" light is still on.

Sounds like a heat probem to me. The computer will shut down to stop the processor from overheating. Will do it randomly in the middle of anything.
Have a look and where the computer takes air in. Is it clogged up with dust? Are you using the computer on a hard surface so there is a gap underneath that air can get past. Are all the 'legs' intact and raising the computer above the table. Do not use on a tablecloth or on your bed or on your lap.

You may have to get someone to open the notebook up and clean the heatsink by the fan properly.