I pressed 2nd and down arrow and the ti 84 turned off and it won't turn on

Press 2ND [TBLSET], then use the arrow keys to highlight "Ask" on the "Indpnt" row, press ENTER then 2ND [QUIT].

If you cannot see anything on the screen, you may need to adjust the graphing TI 84 calculator contrast :

1.To darken the screen, press and release 2nd key, and then press and hold up-arrow key until the display is sufficiently dark.

2. To lighten the screen, press and release 2nd key, and then press and hold Down-arrow key until the display is sufficiently light.

TI calculators have an annoying habit of dimming the screen, especially when the batteries are changed, so that it appears as if the calculator isn't turning on. Turn the calculator on (or at least press the ON key). Press and release the 2ND key. Press and hold the Up-arrow key for a few seconds and see whether the screen becomes legible.
If this isn't the problem, please reply to this post and give a fuller description of the problem.

Turn the calculator on. Press and release the 2ND button. Press and hold the up-arrow button to increase the screen contrast. If you go too far, use the 2ND and down-arrow buttons to decrease the contrast.

Turn the calculator on. Press and release the [2ND] key, then press and hold the down-arrow key to lighten the screen. If you go too far, use the [2ND] and up-arrow keys to darken the screen.

Turn the calculator on. Press and release the 2ND key, then press and hold the down-arrow key to lighten the screen. If you go too far, use the up-arrow instead of the down-arrow to darken the screen.

Turn the calculator on. Press and release the 2ND key. Press and hold the down-arrow key for a few seconds until the screen lightens.

Obviously, if you've gone too far, press and release the 2ND key, then press and hold the up-arrow key.

Turn the calculator on. Press and release the 2ND key. Press and hold the down-arrow key until the screen lightens. If you go too far, press and release 2ND, then press and hold up-arrow.

From TI FAQs:

What troubleshooting can I try if the TI graphing calculator will not turn on?

For the TI-83 Plus Family, TI-84 Plus Family:
Caution: The resets described in these troubleshooting steps will cause a loss of applications, programs and data. If at all possible please create a backup using TI Connect™ first.

1. Try a new set of AAA batteries.
2. Adjust the contrast by pressing and releasing the [2nd] key followed by pressing and releasing the up arrow; repeat as necessary. To adjust or lighten the display, use the down arrow.
3. Remove one of the AAA batteries. Press and hold the [CLEAR] key. While holding [CLEAR], reinsert the AAA battery and then turn the calculator on. The calculator should display the message "RAM Cleared". Release the [CLEAR] key and then press it one more time to remove the message.
4. Remove one of the AAA batteries. Press and hold the [DEL] key. While holding [DEL], reinsert the AAA battery and then turn the calculator on. The calculator should display "Waiting...Please install calculator software now". Follow the instructions for the TI-83 family or TI-84 Plus family to reinstall the calculator operating system.
5. Remove all of the batteries including the round lithium battery for 5 minutes. After the 5 minute period, reinsert all of the batteries and turn the calculator on. Adjust the contrast if necessary.

Press the 2nd key (yellow one) and then press the UP arrow to increase brightness or down to decrease but seems like you to need to press UP arrow.