Contact the plan administrator for the 401(k) plan and ask for the 1099-R.
A rollover from 401(k) to an IRA is required to be reported on a 1099-R (unlike an IRA to IRA rollover).
You want to find out if the 401(k) plan administrator issued a 1099-R and if your copy was simply lost.
Since the 1099-R is copied to the IRS, it is important that you know what was reported to the IRS.