Your bike currently has a functioning clutch lever (the thing on the handlebars) and functioning clutch release mechanism (the arm on the back of the gearbox + a bunch of internals). However, pulling on the clutch lever has no effect - you're constantly in gear (the rear wheel will still not turn). Put away the holy water and stop praying, it's not demonic possession - it may be that you just have a stuck clutch.
I've discovered plenty in the last while about the clutch and its correct adjustment. Didn't see it in 'tips and tricks' so here's how I solved my 'stuck clutch'. This was necessary after just three weeks of not riding the R90/6.
Ensure you have the correct adjustment. This is a worthwhile read and probably worth looking at prior to Step 2.
Clutch adjustment (Pre-1981) & How to ride with no clutch cable
Fixing a stuck clutch
1. Get into neutral (you may need to manually rotate the rear wheel back and forward, while clicking away at the foot gear selector).
2. Start engine. Let her warm up.
3. Don all safety gear (in case this doesn't go to plan).
3. Grab the clutch. Don't let it out again.
4. With engine running, and bike in neutral, and while holding the clutch lever, get the bike moving (you pushing, friend pushing, rolling downhill - like you were bump starting).
5. The engine is running, so when you're moving at a reasonable speed (same speed as you'd ride in 1st) 'stomp down' it into 1st. This might not be healthy, but your bike will do as it is told.
6. Now you're in 1st gear and you're still holding the clutch, but the bike is in gear and you're moving. 1st might not be enough to solve a stuck clutch, but 2nd should be.
7. A little bit more throttle and 'stomp up' into 2nd.
8. With any luck you're now riding a bike that has broken it's clutch free, and because you're still holding the clutch, you're starting to slow down.
9. Time to let the clutch out and ride around for awhile. The solution to stuck clutches is regular use. Be good and ride regularly.
N.B. Between 4 and 7 you may look like you've had your license for less than a week. Probably a good idea to put on a fake mustache and dark sunglasses before you get started.
N.B. All work done occurred while under the influence of home brew ginger beer. I'm not say that's why it was so easy, but it might have helped.