I can lock or unlock it from the inside, where there's another key. On the outside, though, I was having trouble locking it, so I sprayed it with graphite. (Well, it didn't warn me on the bottle NOT to do that!) Now I can't turn the key in either direction. At least there's another, different lock underneath, so I use that for the time being.
Granted, it's been a little humid lately, but I never had this problem in the last 17 years or so.
As for it working great in the past, I get that... I actually specialize in antique locks... and years ago in most areas graphite was a great CHEAP solution for lock lubrication... The problem is as machining tolerances have tightened, thanks to using CNC even in mid to high grade residential and commercial locks (standard security, and ESPECIALLY high security locks) instead of standard hand machining or casting there's more of a chance that the graphite will do more harm than good.
Medeco is a prime example of having tight tolerances, a complicated mechanism that needs to move freely, and a lot of places for dust or graphite to get gunked up in fouling up the lock... So graphite in Medeco is just asking for trouble...
The other thing is, these days there's better stuff out there (like the TriFlow, ZEP-45NC, etc) even for older locks, that DOESN'T get all over the place, stain everything, and jam newer locks up if too much is used that is not much more expensive... Graphite is mostly obsolete in most cases involving residential or commercial structural locks (it's even being phased out in ignitions and other electrically sensitive keyed switches where a liquid spray is not recommended). Trust me on this one, your locks will work better, be better protected against corrosion, last longer, and your locksmith will be happy to not have to deal with it when you do need to call them.
I'm glad the situation was resolved... I still STRONGLY URGE you to flush the lock clean of any graphite and use a different lubricant on modern, especially high security locks... It's only a matter of time until things gum up... Bear in mind, I never said that graphite will "damage" your lock... Medecos are TOUGH.... they're made to resist actual damage (unintentional OR intentional) but it WILL gum it up... Again consider what would happen if someone put elmer's white glue in your lock... It's not "damaged" it could be flushed out with hot water.... it just won't function until it's cleaned out.
I didn't know that you kept the inside key in all the time, otherwise my recommendation would have been slightly different, and would have suggested trying the "inside key" on the outside... before recommending you flush out the lock....
So it seems your "outside key" is just worn out.. Still though, you should flush that thing out. I've been a locksmith for over 15 years, and deal with Medecos monthly, and it's the FIRST thing I would do...
Whatever you decide to do in terms of graphite, all you need to do to return things to normal (most likly) is order another key, using your Medeco card either direct from Medeco or from a Medeco authorized locksmith and you'll have two working keys again.
As it happens, I tried it again and I was able to lock-unlock from the outside, even when the inner key was inserted. But I decided I didn't want to take the same chance again, so I took out the inner key anyway.
How DO you flush a lock with hot water, since even an eyedropper wouldn't fit? Also, how do you make sure it's dried properly?
Use a 50CC syringe, AKA lavage syringe, and small hypodermic needle to flush it with hot water. Use the same set up to flush it with isoprophy alchol a couple of times, ten minutes apart. Wait about 20 minutes and dry it with a hair dryer.
OK, I posted the "hot water" comment as an example of how your lock isn't "damaged" it's just gummed up... while could work, the easiest and best solution is still take your lock off the door and flush it out with a cleaner solvent...
I would recommend Tri Flow or Zep 45 N/C... Both available on Amazon if you can't find them in your local hardware store. Make sure you're not wearing nice clothes for this, graphite stains... Just keep spraying and draining through the keyhole until it starts running clear... Let it drain for a bit, wipe it down, and put it back on.
If you don't want to go through ordering online, you COULD use WD40, which is available everywhere, but it's gonna take a LOT longer, so buy the biggest can you can find (thankfully it's dirt cheap)
Also, if this is a Medeco Deadbolt, DO NOT USE Houdini Lock Lube neither. It is not designed for the Medeco Deadbolts. It will push back the gunk to the back and lockup the deadbolt completely. If it is a Medeco Deadbolt, use something very light such as TriFlow as mentioned above.