Camera not working

My old Canon 20D will only show "Busy" on the LCD when I press the shutter button....any ideas why?

"Busy" normally means the buffer is full and a write operation to the memory card is occuring. If the card is corrupted for some reason you can get a continuous "busy". I would attempt to remove all the images on the card with a card reader. Then do a format of the card, reinsert in the camera, do a low level format and retest. An alternative would be to test with a different card.

Were you using a grip? There have been known contact problems with vertical grip. There's a small switch where the grip meets the body that can stick making the body think the battery door is open. See if you can locate the switch and make sure it moves freely, then test without the grip.

I'd flick the terrible "shoot without CF possible" option to ON and see if the camera operates more-or-less as it should (then quickly put the option back to off...)

Usually a loose grip just lets the camera die unexpectedly... no power, nothing. So I doubt that's the problem.

I'd also suspect the memory card first. In addition to trying a different card and formatting it (be sure any images you want have been copied off first, fformat in the camera, not in a computer)... also remove the card and inspect the pins in the memory card socket closely. Look for any bent or broken both inside the camera and the end of the card.

If you see a bent pin, don't insert another card in the camera and risk bending it further. Some pins, if bent into contact with other metal parts, might cause an electrical short and damage the circuitry. If that happens, it probably would be too costly to repair the camera.

There is also a microswitch on the memory card door. Check that's not stuck and the door is closing fully.

Other than that, you might try a "reboot" of the camera, which is a lot like rebooting a computer to clear the memory of any fragments or errors. To do this, first turn off the camera and take out all the batteries... both the rechargeable and the silver CR2016 date/time lithium battery that's in a little drawer next to it. Turn the camera on and press the shutter release button once. It won't fire, but this should drain any remaining power in the circuitry. Turn the camera off again, reinsert the batteries, turn it back on and check the date/time... Those should need to be reset, if you got a proper reboot. If you did, great. Try shooting with the camera again. If date/time weren't lost, then you didn't get a proper reboot and should either repeat the process... Or just leave the batteries out for a lot longer time, with the camera's main power switch set to "on"... Minimum one hour, or up to overnight... to get a proper reboot.

The only other thing you can do is try reinstalling the firmware in the camera, if you can get it to function enough to allow that. You have to be able to get into and navigate around the camera's menu. You can find the firmware on Canon's website and download it, follow the instructions there very carefully to install it. This might fix it, if some of the firmware has gotten corrupted. However, if the camera is only working partially, it might not be possible to update/reinstall firmware yourself.

The above are things you can try yourself at no cost. Worst case, none of those work and something in the circuitry isn't allowing the camera to write images to the card... Might be as simple as a loose connection or more complex such as a failed component. Howebver, usually when these occur you get an error message on the camera, of some sort or another. If it's anything more complicated than the DIY fixes mentioined above, the camera will probably need repair at Canon's or some independent repair facility.

I bought another CF card and stuck it in the camera. All is working now.....

Take that old card and do a format with a computer. It may very well be salvageable and if not depending on the brand may have had a lifetime warranty.