You need to reinstall your OS.
1). You can get the latest copy of Ti84 Plus 2.55v. MP OS and, guidebook by going to the Texas Instruments web site. You can get to that site instantly by clicking, "Ti 84 Plus 2.55v. MP OS!" Once your at the Ti site click the Download Item, "Ti84 Plus family operating system". Next you can loginor, click "continue as guest". I'd would click the"continue as guest". Once you have click, "continue as guest" the Ti84 Plus family operating systemwill download to you.
Next, click "SAVE" and then a window to a folder will pop up. It is probably a window to a folder that you download too. If so, you may want to change it and, save to this folder, "My Documents \ My TI Data \ Downloads". You can also go back and, download the Ti 84 Plus guidebook too, if needed.
Link your computer and, Ti 84 Plus calculator with the USB cable or, Direct Black Link cable that came with the Ti 84 Plus calculator. Next, drag the Ti 84 Plus 2.55v MP OS that you saved earlier in "My Documents \ My TI Data \ Downloads" out of that folder and, drop it in the "Ti Connect" icon on the desktop. Click, "Continue" when the "Erase Warning" pops up and, the new Ti 84 Plus 2.55v MP OS will begin to install.
Next, the Ti Device window should pop up and, highlight the port when it finds what your Ti 84 Plusis using, USB1 cable or, Direct Black COM1.
When the Ti Device Explorer finds and, highlights your calculator's information click, "Select." Then click, "Yes" when the next window shows up with your calculator's information and, the OS will continue to download. Upon completion the newestTi 84 Plus 2.55v. MP OS will be installed.
Source : http://www.fixya.com/support/t7767740-when_ever_turn
