Normally it is not recommended to borrow money from your retirement accounts unless you absolutely need to. Last night on the Suze Orman Show, Suze gave a few ways to borrow from your retirement account if you need cash and have no other source for it.
If you have a Roth IRA you can borrow any of the money that you have personally put into it without penalty or taxes.
If you have converted a traditional IRA or 401K into a Roth IRA, after 5 years you can withdraw any amount of the amount converted without penalties or taxes. Any growth on that amount needs to stay in the account for another 5 years or until you are 59 1/2.
If you have a traditional IRA, you can borrow from it, but it needs to be repaid withn 60 days. So this is only an option if you need cash for the short term.