This bug/issue has been present in NEXT as well and I don’t think a solution was ever found, nor a cause that reproduces it.
Frigates that are out on a mission can sadly enough not be dismissed. Your frigate is most likely damaged, not being able to continue, hence the timer stopped. You are supposed to be reported about the issue and make a choice, risk losing it by having it continue the journey, or call it back for repair. Most likely you never received the report, causing you to have it stuck without an option to solve it.
Only possible solution I can think of, is trying to locate the frigate. Then hopefully be able to repair on the spot, to have it continue the mission. No guarantee this will work though.
Also see this previous NEXT topic with same issue: