I think the only way is to format it or connecting it to a computer. I tried searching for solutions on Google and could only find Nspire-related stuff.

Oh wait I thought you had a 84+ for some reasons, since you talked about having to connect it to another calc. In that case, this could be useful, although it's in French https://tiplanet.org/forum/viewtopic.php?f=57&t=6450 . It's a tutorial to get out of Nspire's PTT mode.

Using Google Translate to make your homework is not a good idea but it works fine in situations like this one. Here is what it gave me.

The PTT mode with Norse unlock the LED / LED of the Nspire (except Nspire gray ) . But getting out is not as simple as entering : you need to create an empty file on the computer ( using the TI- Nspire software corresponding to the OS of your calculator or earlier) that you call "Exit Mode.tns test "(without the quotes). You must then send this file to your calculator, in the " Press To Test Mode" ( already created upon activation of the PTT) . After a reboot, the PTT is disabled! So a quick summary :
1) Install the TI- Nspire software for the version of the OS contained in your calculator (if already done so, go to step 2)
2 ) Launch TI-Nspire and save a blank document and name Exit Test Mode.tns
3) Send this file in the "Press To Test Mode"
4) Your calculator will restart , and the PTT is disabled!
Because the file can be 1kb ( it must be of 0kb , it is very important) , you will find downloadable ZIP file at the end of this tutorial . So you will only transfer using TI- Nspire Computer Link.
Download file Exit Test Mode.tns

Except the words "folder" and "have to" missing after "Press To Test Mode" and "you will only", and apart from the first sentence meaning nothing, I think that the instructions are really clear.