Open it up, look for limit switches at both ends of normal travel. Use a mutimeter to check the extended limit switch. Check C to NO, then C to NC. One of them should read 0 ohms (closed), the other infinite resistence (open). If they both read open (or less likely both closed), the switch has failed internally. Replace the switch you should be good to go. If the contacts check out as normal, look for a mechanical fault that is keeping the swithch from being actuated. Could be as simple as a broken or missing roller on the end of the switch lever, or an attachment on the drive rod that's supposed to engage the switch but is not doing so.
If you have no luck with that, try this guy: