For 2017 I'm filling out my Schedule C and my mileage log is missing.  I drove several thousand miles and cannot afford to lose this expense?

If you tell your software you don't have mileage records, it will not include it as a deduction. If you deduct it and get audited, you would likely lose the deduction and have to pay penalties and interest. 

Your best bet is to use actual expenses this year. It may not save you as much in taxes but it will withstand an audit, although its possible it will be more advantageous in tax savings.

If you used the standard mileage in previous years for the same vehicle, you can use actual expenses this year and switch back next year. There are some restrictions on going back in forth but only if you depreciated the vehicle or deducted the entire cost previously. 

You can LEARN that such records should ALWAYS be kept in a form you CAN'T lose when you leave a job.

Legally, you are required to have proof of the miles or you don't qualify for the deduction. But you don't have to show those records to anyone until/unless the IRS audits you.

If you tell the software that you don't have records of the mileage then the software won't claim the deduction on your return.

If you tell the software that you do have records, then it will claim the deduction for you but you don't have to enter those records or include them with your return. You just have to be able to show them IF the IRS audits you and asks for them.

Proof isnt required until there is an audit.  

If you check the "no" box, they will disallow the expense. Written proof IS required.

Unless you're audited, the proof won't be needed. It's not the other way around.

Can you go back to the old account and have them print off that info for you?

You must have the log. Simple. If you do not, they will disallow the claim, especially when you try to deduct thousands of miles.

Then you have a problem. You have no log and no work diary to construct one from.

The IRS requires STRICT substantiation of all vehicle expenses/mileage. No docs, no deduction. Sorry.Your ONLY hope is to get a copy of that email or its equivalent.  Contact your old employer and ask for help.When I traveled for work, I had to turn in time sheets listing where I'd been and what work I'd been doing.  See if your boss still has those.Note, no one should recommend playing the audit lottery.  While you may see news items saying that IRS audits are low, this is not necessarily true.  Schedule C filers are audited more than those who have W-2s, especially if you deduct anything or can claim EIC/ACTC.Note I see a lot of people saying it's no big deal to lie.  When you get audited and can't produce the log or the diary, the IRS will not only slap you with taxes, failure to pay penalty and interest, it can also slam you with a 75% penalty of the amount owed for fraud.