My SanDisk clip mp3 won't turn on at all.it only

Disconnect the player from the Computer then press and hold the Reset button on it for 30 seconds to reset all the settings to the factory default.

Then, connect the player to a computer to charge it completely. Once the player is charged sufficiently then try turning it ON.

If the MP3 player still doesn't turn ON then the LCD screen is required to be replaced.

Possibilities include that it's not actually charging properly and is now so very very flat that it can't stay on at all (check your connections, swap your usb port, cable etc). Otherwise, it may be, that the software is corrupt, or that the hardware is physically damaged.
Does it stay turned on when you remove it from the computer connection? If so, take the chance to reformat it (you'll lose whatever's on it now). Plug it back it, and update your software via the sansa updater programme ...
But, if the switch is simply stuffed this won't help!