Do i have to file my 401k? I didn't cash it in.

[Bsch4477]

You do not have to report any activity within your 401-k.  You only report any distributions.   

[DoninGA]

If you did not make a withdrawal or a roll-over from the 401(k), then you have nothing to report on a tax return concerning the retirement account.

If you contributed to the 401(k) that amount is on your W-2 in box 12 with a code D.  The contribution is not entered anywhere else on a tax return.