Spacebar on my Microsoft Natural Ergonomic Keyboard 4000 is sticking in the down position.

SOURCE: REPLACEMENT PART

I don't think they sell keys, take one from another keyboard or replace the keyboard.

SOURCE: My Keyboard won't type on Word program.

It seems like your trying to type into a locked document, which means that your keyboard is working, its not the issue, the issue is the spesific file which is locked, try another document / file or turn off the lock from office settings if you can find it.

SOURCE: Microsoft 4000 Ergonomic keyboard - broken foot

they don't really make replacement parts for these, its not profitable just a small piece of plastic, you would have to create your own from metal or similiar, or use the keyboard without the feet, or replace the whole keyboard.

SOURCE: the spacebar brings up help info

you need to remap your keyboard. ask Windows help for complete instructions.

Testimonial: "is remaping when you look at the language and sort that out because that is what I have done but it still has the same problems"

SOURCE: Where can I get the keyboard cable that connects a

try ebay or microsoft shop however it may be cheaper to replace the keyboard.