Anne-
I am not familiar with Toshiba model designations but I think the Satellite 
is a laptop, if not please disregard the following.
You didn't say whether you were operating on the battery or ac adapter when 
this occurred.
In addition to what Tom suggests I suggest the following.
If this occurs when operating on battery  it may be your battery is loosing 
its charge capacity. As a battery ages it loses it's ability to supply it's 
rated current capacity. Many current electronic devices have circuitry that 
automatically shuts the device down when the battery discharges to the point 
it cannot supply enough energy to properly operate the device.
If this occurs while operating on an ac adapter it may be a problem with the 
power supply. Ac adapters have filter capacitors in them that over time dry 
out which reduces the dc output and causes more ac ripple (less DC) in the 
output. Many laptops are very sensitive to ac ripple in their power source.
If this problem occurs in both modes, AC and Battery, you probably have 
other problems that a Tech would have to address.
If you have them available you might try substituting a different battery or 
ac adapter.

HTH
John Dinges 

               The NOSPIN Group Promotions is now offering
                 our special coffee mugs and mouse pads
              with the PCBUILD logo...  at a great price!!!
             http://freepctech.com/goodies/promotions.shtml