Your employer can withdraw money from your 401 (k) plan after you leave the company, but only under certain circumstances, as explained by the Internal Revenue Service (IRS). Depending on the facts and circumstances, your plan may have a partial termination. This can happen if an employer action causes a significant (usually at least 20%) decrease in participation in the plan. Layoffs, changes to the plan, or business reorganizations that cause a decrease in participation in the plan are counted even if they are due to economic circumstances beyond the employer's control.
See the plan termination FAQs for more information. Generally, a 401 (k) plan doesn't tax investment gains until you decide to withdraw the amount from your account. This usually happens after you retire, as you won't always be allowed to withdraw any amount before you retire. However, when it comes to Roth 401 (k) plans, withdrawals aren't taxable.