The word/verb, παραπεσόντας (from παραπίπτω) is an aorist active participle, masculine plural; literally fall beside or aside, go astray, become lost; figuratively in the NT of abandoning a former relationship turn away, commit apostasy, Hebrews 6:6.
The aorist tense is often translated in the past tense,and this word is an aorist participle. The aorist tense often denotes a past event captured in time, sort of like a snap shot pic taken by a camera that captures the motion of an event as it happened. Wallace argues that the participle, παραπεσόντας, should be taken as adjectival, and translates this text as “It is impossible to restore again to repentance those who have once been enlightened . . . and have fallen away.” See Daniel B. Wallace, Greek Grammar Beyond the Basics (Grand Rapids: Zondervan, 1996), p. 633.