The answer depends on whether you terminated employment prior to meeting the plan’s eligibility requirements or after you had become a plan participant, and will also depend on the employer’s adopted plan document.
Generally, if you terminated prior to satisfying the plan’s eligibility requirements and are re-hired prior to the time the eligibility requirement’s initial computation period expires (12 months beginning on the original date of hire, if a one year of service requirement applies) your prior service will count towards satisfying the plan’s eligibility requirements. If re-hired after the expiration of the plan’s initial eligibility computation period, but prior to 12 months following your termination date, you will be required to satisfy the eligibility requirements using the plan’s plan year as the eligibility computation period.
If re-hired after having met the plan’s eligibility requirements, but prior to actually becoming a plan participant (you were not employed on the plan’s next entry date after satisfying the eligibility requirements) you will become a plan participant immediately upon your re-hire date.
If re-hired after having met the plan’s eligibility requirements and actually became a plan participant (were still employed as of the plan’s next entry date) you will become a plan participant immediately upon your rehire date.
This question is a very technical one and each employee’s situation will be different depending on a number of factors. Contact your plan’s Plan Administrator for further clarification if this situation applies.