You will lose eligibility to establish or contribute to the HSA. You can keep your HSA and continue to use the funds on a tax-exempt basis for qualified medical expenses. However, you are prohibited from contributing to the HSA or establishing another HSA unless you become eligible again.
If you plan to enroll in a QHDHP in the future, you should contact Highmark to change the status of your HSA to establish eligibility for future contributions.