Why is my calculator turning off by itself?

SOURCE: calculator keeps powering off after a few seconds

1. Remove the keypad (for TI-Nspire) or the battery door (for TI-Nspire CAS) and remove at least 1 battery from each column of batteries. After 2-5 seconds, reinsert the batteries and turn the unit on. This will cause the unit to reboot the operating system.
2. Hold down [Home] [Enter] [P], and while holding these keys, press [On]. The device will boot up as in the previous reset, but will display a menu with the following options. Please try options 2, 3, and 4 in that order.
   
   1. Cancel - The most non-intrusive reset option; no changes are made, the unit restarts. 2. Delete Operating System – The operating system will be deleted without deleting any documents. 3. Delete Document Folder Contents – This will delete the user’s documents, but will not delete the OS. 4. Complete Format – This will delete the OS and all documents for a total erase of memory.
   Note: TI-Nspire family model options 2 and 4 above require that the OS be reinstalled. The reset menu will only display if the unit is off and batteries have been removed/replaced as described above.

SOURCE: my ti-84 calculator show unreadable code only?

Honestly, I do not understand what you mean. This calculator is designed to run commands one after the other. You may stack a few of them only if you do not need the output of the first ones.
Anyway, the memory is limited and that includes the register where the current line is stored. If you enter too many characters, the calculator stops accepting them and the cursor will change shape.

If memory is full you can no longer do anything. One simple way to regain some working memory (RAM) is to clear entries (the commands you typed). Press [2nd] [+] to enter memory managem2nt and select the command ClearEntries. You can also select ClearAllLists, or Reset. For the last choice you can then choose to reset ALL RAM.