What happens to your 401k when you are fired? How do I access the funds?

Very sorry about your job loss…hopefully that’ll just be temporary. It looks like your question focuses on drawing out the funds, but I'll list various options in case:

1) leave the 401(k) account with your former employer

2) temporarily leave it with your former employer and then—when you land a new job—transfer the vested balance to the new employer’s retirement plan (if new plan will accept)

3) directly roll the vested balance to an IRA, or

4) withdraw some/all of the vested balance and pay income taxes and 10% federal penalty tax if you’re under age 55

Regarding #4, folks age 55+ avoid the 10% federal penalty tax on distributions from a 401(k) when they terminate employment at 55 or later. That's in contrast to the applicable 10% penalty tax until age 59.5 if the distribution comes from an IRA. 

Procedurally, you'll want to contact HR or the 401(k) plan administrator for forms authorizing either 401(k) withdrawal(s) or rollover to IRA. The time it takes varies depending on the plan sponsor and custodian. You are always 100% vested in any contributions you made through payroll. There may be a vesting schedule for matching/other contributions your employer made on your behalf.

If you don't absolutely need the funds now, directly rolling it to an IRA with a discount brokerage firm will allow it to continue (potentially) growing tax-deferred and give you a greater level of control over investment choice and expenses. 
I hope that helps. All the best!