If the 401(k) rollover is the only amount in your Traditional IRA and if you only have one IRA, then your basis is zero. Even though you did not deduct the rollover on your tax return, it is not considered a nondeductible contribution for basis purposes. Of course, I'm assuming here that your 401(k) plan came with the pre-tax dollars, as it happens with the most of the 401(k) plans.
Please read the article onTraditional IRA Basis, it has a nice explanation of your situation.
As to the Form 8606- if you have any nondeductible contributions that you made with the after-tax dollars, it's not too late to file the form. You can file it by itself - meaning you can file it/them without amending your tax return. Please note you must file the form for each year that you made nondeductible contributions. There may be a penalty ($50) for not filing it timely, but the penalty may be waived if you can show a reasonable cause for not filing. Here are instructions for Form 8606 and Form 8606, and see this information about the reasonable cause.