"The first thing to do is make sure it is full of fluid. If so, then is there a chance the filter has fell off, and its not picking up fluid? Look under the vehicle to make sure the shifter is moving the arm on the transmission. Your shifter may have broke somehow to cause this. Does the transmission just whine when you put it into gear, or is it even going into gear? You have got to make sure the shifter is moving the arm on the transmission. In the worst situation, you may have a broke pump, as this will go out all at once, and the vehicle will have no gears at all. It is common for a filter to not be on good, and it fall into the pan, thus can't pick up any fluid, and it won't move at all, but you will hear the pump whine if this is the case. Pull the dipstick out, and see if you can see any metal glittering in the sunlight on the stick. Get the engine hot, with the vehicle on level ground, check the fluid level with it in park. If there is none showing on the stick, add a couple of quarts to see if it then goes. If it is empty it might take as many as 3-4 quarts to make it go. I'm not sure how your shifter works, but if it's one that has a cable that works the gears, then it could have broke, and you would have no gears, just what ever its stuck in. I can't stress enough how important it is to not get under a vehicle unless you are 100% sure it can't fall on you, and make sure it won't all of a sudden go into a gear and run you over. You can never be too cautious, as these will kill you if you're not careful. Another common mistake with these is; someone will change the filter, and not get the old grommet out of the valve body, and put another one on the new filter, and when they push the filter into place it will just suck air and no fluid.
You are not the first one to have this problem with the B&M shifter. There are two types of shifters, one that works by linkage, and one that works by cable. There's not anything that would get caught in the shifter unless you have a light in the shifter, and the supply wire is caught in it somehow. If you can put the vehicle on a rack, or jack stands, and have a friend work the shifter while you watch what it's doing. I think what has happened to yours is; the cable has snapped, thus you don't have any gears. The cable can also slip out of the retainer, and not move the arm on the transmission. Without seeing what is going on, no one can tell you for sure what is, or isn't working. You will just have to see if it's moving the arm. If its not moving at all, then you can bet the cable broke. Have you discussed this with the technical support line at B&M? Try to remove the shifter from the transmission shift arm, and see if you can manually move it from one gear to the other. If so, then you have rulled out anything with the transmission. Outside of this, there just isn't anything else to tell you to do."