First try removing all four AAA batteries and the small lithium backup battery (above the four AAA batteries). Leave all batteries out for five minutes, then replace them and try to start the calculator. If this doesn't work...
Replace all but 1 AAA battery. Press and hold the "CLEAR" button on the calculator. While holding the "CLEAR" button down, replace the missing AAA battery. While still holding the "CLEAR" button, turn the calculator on. The display should read "RAM Cleared". Release the "CLEAR" button and press again to remove the message.
If that doesn't work, remove 1 AAA battery again. Repeat the preceding instructions with the "DEL" key: With the battery removed, press and hold the "DEL" key. With the "DEL" key held down, replace the battery and turn the calculator on. The screen should say "Waiting... Please install calculator software now." Release the "DEL" key. Download this
program onto your computer (from Texas Instruments). Connect the calculator to the computer using the USB cable provided with purchase. Run the TI OSDownloader program and follow the on-screen instructions to re-install the calculator's operating system.