For as much trouble as it is to open the case without doing more damage, I recommend the following:
1. try adjusting the contrast to make SURE it is dead.
2. Press on the keypad just above the C key while you turn it on.
3. Repeat step 2 for six other keys ( O,Y,A, F, 7, 9)
4. Sell it for parts and use the capital to buy another.
On many models other than the 48sx #4 would be a last resort, but unless you are unemployed or bored (or both, SMILE) , it is probably not worth your time to repair it when they can be had on TAS (that auction site) for $60 to $120 in working condition, while $20 to $35 for parts.
Otherwise, I have several 48g calculators with this condition. Using a DC powersupply and ammeter I can determine that the calculator is ON and responds to keypresses, but the display is bad. You can tell based on the current draw in different modes. The processor draws much more current when servicing interrupts (such as keypress, running programs, RAM access and IO) than when only powering the LCD and clock.
Edited: 15 Jan 2007, 3:45 p.m.