While you may no longer contribute to your employer sponsored 401(k) or 403(b) plan once you leave the company, your employer, in most cases, is still obligated to maintain it for your benefit. There are some exceptions. If your company retirement account has less than $1,000 in it, your employer may close it out and send you a check for the balance. If your account has more than $1,000 but less than $5,000, your employer can automatically roll those assets into a "Safe Harbor" IRA at a provider chosen by the former employer.
If you are interested in continuing to build your retirement savings after your leave your employer, you can rollover your funds to an Individual Retirement Account (IRA) or possibly to your new employer's plan.