My TI-84 won't turn on. Have already replaced all the batteries and even the back up one. I also tried adjusting the contrast but nothing happened.

If the TI-84 does not function even though you are sure that the
batteries are fresh, you can try manually resetting it by following steps:

1. Remove all of the AAA batteries from the graphing handheld

2. Press and hold the ON key for ten seconds

3. Replace the batteries

4. Turn on the unit

When you reset your graphing handheld, the contrast sometimes
changes. If the screen is faded or blank, adjust the contrast by
pressing 2nd and releasing up-arrow or down-arrow keys.

If the TI-84 SE does not function even though you are sure that the
batteries are fresh, you can try manually resetting it by following steps:

1. Remove all of the AAA batteries from the graphing handheld

2. Press and hold the ON key for ten seconds

3. Replace the batteries

4. Turn on the unit

When you reset your graphing handheld, the contrast sometimes
changes. If the screen is faded or blank, adjust the contrast by
pressing 2nd and releasing up-arrow or down-arrow keys.

Change the battery

Having gone over two months without a response, I assume my suggestion to adjust the screen contrast solved the problem.

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.

Could be that your calculator needs to be reset. Take out the internal battery that is located in a tiny door with a screw right above the 4 AAA batteries. Leave this out for about 8 hours and put it back in. Put in the AAA batteries and the calculator will be reset and may work.

try adjusting the contrast up using 2nd + up
just keep pressing it and it should turn back

This is kinda a weird situation. Because it's usually the other way around. Your ram will clear if your calculator is frozen. So take out all 5 batteries from the back. Even the back up battery. Then reset it again. 2nd then + then 7 then 1 Then 2. Good luck.

Try the following

1) Remove one of the AAA batteries, hold down on the DEL key while reinserting the AAA battery. The handheld might display Waiting…Please install calculator software now. Ignore this message. Remove and reinstall one of the AAA batteries again This time, do not hold down the DEL key.
2) Remove one of the AAA batteries, press and hold the [on] key for 10 seconds, release, replace the battery, then turn the unit on. Adjust the contrast.
3) Remove the AAA batteries for three hours. After the three-hour period, reinsert the batteries and turn the handheld on. Adjust the contrast if necessary.

If needed, reinstall OS by downloading the OS from the TI site and ....
On your computer, choose Start, Programs, TI Tools, TI Connect, TI OS Downloader.

(I am not the one who came up with this answer, but found it on an other site. If this doesn't work, please let me know)

Gr Marijn

Try the following

1) Remove one of the AAA batteries, hold down on the DEL key while reinserting the AAA battery. The handheld might display Waiting…Please install calculator software now. Ignore this message. Remove and reinstall one of the AAA batteries again This time, do not hold down the DEL key.
2) Remove one of the AAA batteries, press and hold the [on] key for 10 seconds, release, replace the battery, then turn the unit on. Adjust the contrast.
3) Remove the AAA batteries for three hours. After the three-hour period, reinsert the batteries and turn the handheld on. Adjust the contrast if necessary.

If needed, reinstall OS by downloading the OS from the TI site and ....
On your computer, choose Start, Programs, TI Tools, TI Connect, TI OS Downloader.

(I am not the one who came up with this answer, but found it on an other site. If this doesn't work, please let me know)

Gr Marijn