A hardship withdrawal must occur between a qualified retirement plan and a plan participant who is still employed under the company.  The participant must demonstrate that they have an immediate financial need which has resulted from an unexpected event.

First,  it is important to note that plan administrators do not have to accommodate hardship withdrawals.

Moreover,  administrators who provide for hardship withdrawals in their plans get to pick what qualifies as hardship.

Typically the following two situations are accepted:

- unforeseen,  non-reimbursed medical expenses

- mortgage payments to prevent foreclosure upon one’s main place of residence

Age restrictions may also apply,  especially if the plan is a Pension plan.  Under IRC 401(k) regulations hardship withdrawals may not occur until the participant reaches Normal Retirement Age (NRA). NRA,as defined by each individual plan,  can be found in the Summary Plan Document.  If NRA is before the age of 59 1/2,  then the withdrawal is subject to an additional 10% income tax.  The withdrawal amount must also be reported as ordinary income for the individual.
As you can see,  Hardship withdrawals have rigorous qualification standards.  It is very important for all plan participants to be familiar with their Summary Plan Document (SPD) so they know what they are entitled too.
Of course,  if you do not qualify for a Hardship withdrawal you should see if your plan allows for participant loans,  which are much more easily obtained.