We had a similar situation with an employee at my job. For some reason, his insurance premiums had not come out of his check for several months. The benefits administrator fixed his account and put through a deduction that would bring him up to date.
Unfortunately, bringing him up to date meant taking his entire paycheck for about a month. When I questioned it, I was told that his insurance premiums are NOT considered a garnishment (they are a voluntary benefit he signed up for) and were not subject to the maximum garnishment restrictions.
They told me that since he had signed up for benefits, he knew how much was supposed to be coming out of his check each week. He never notified us that it wasn't being deducted properly.
In the end, the payroll manager and I had to allow the deductions to go through so that his insurance coverage would be current. To be kind, my manager advanced him same the amount from the payroll account and we let him pay that back in smaller weekly increments over time so he could pay his bills that month. It was bending some rules but we felt we needed to take care of our employee. In my company, the payroll department has absolutely no control over benefits deductions.
In this case, it was legal for the company to take his entire check. It wasn't right but it was legal. Raise hell with your HR and payroll departments---they may be able to help you out a little. It's not fair to take someone's entire pay. Maybe they could let you cash out some vacation or sick time to cover this bill. Maybe they could give you a small advance. Get creative and be persistent. (I can't tell you how many phone calls I got over the situation I described at the top---but we found a way to get the employee's insurance current and get him a paycheck)
years of payroll work