My sigma 24-70mm f2.8 doesnot work on my d7000

Turn the aperture ring on the lens to its smallest setting (largest f/number) and then forget it's there. You can control the aperture from the camera body, the same way as on lenses without an aperture ring.

If you're worried about the trigger voltage, that's not a problem. However, you may not get all the features you'd get with a more modern Nikon flash.

The trigger voltage of the Sigma EF 430 is safe for use with the Nikon D200.

Do you mean the EF 430? If so, then yes, it is compatible with the D60 and D7000, at least as far as the flash trigger voltage is concerned. You may not get all the capabilities of more recent Nikon flashes, but it will work.

this flash is not compatible with the d200. i had one and sent it back to sigma, they upgraded the software in the flash for free. now it works in all modes, it needs to be iTTL to work on the nikon d200 the NAd version is not iTTL.

This Sigma flash is not Ittl compatible. The D300 and D200 Nikon must have Ittl compatible flash to operate in any of the Automatic modes. It is a great flash but will not work on those two cameras.

Have a close look at the iris ring to see if any think is broking off
or try moving the lens when it is locked on the camera to see if you lose the fee if so it may need a new mount

I'm assuming this lens worked on another Nikon camera before and that the lens mount is for Nikon cameras. Make sure the aperture ring is set to "A". Clean the contacts on the lens and camera body where they meet with a cotton swab lightly dampened with rubbing alcohol.

Sigma lenses sometimes need re-chipping for full compatability with certain Nikon camera. They are usually quite good about doing this gratis.
Try the Sigma on another body at the camera shop or at home (preferably and older AF film camera). If it works then its likely just a chip compatibility issue.

You won't be able to use older TTL flashguns with the D200. Since the D70, all nikon cameras have used "iTTL", and earlier DSLRs (eg D100) have used "D-TTL". Neither of these are compatible with the older TTL system used on film cameras. The newer SB-400/600/800 flashguns are all ok, and Sigma and Metz both produce iTTL compatible flashguns. The new flashguns will work on old cameras but not vice versa.

I know this is annoying, but there were good technical reasons why Nikon changed the TTL standards.

If the 636 has an A mode (non TTL) this should work ok, otherwise it will only work in manual (if it has a manual mode).