2000 ZX3, if the key won't turn your ignition switch is trashed. Well known issue and pretty common problem, requires a tow to the dealer to fix. don't know if it's a recall or TSB you can get fixed for free or if they'll charge you a lot of cash to fix it.
It was really common when they were only a year or two old! It was happening back in '01 and '02! It's usually accompanied by the key getting stuck in the ignition. Is it stuck?
If it isn't stuck (i.e. you can pull it out), first try a second key (you have a second key, right?) and fail that, call a locksmith instead. your key might be worn out or something. If you have a lot of stuff on your keychain, it can wear out the ignition cylinder or key a lot faster. A locksmith might be able to cut you a new key or re-key your existing lock.
If it came out with difficulty, then it's the same issue.
Try graphite powder. a few puffs of it at a time, then work the key in and out. Then repeat until you have about 10 puffs of graphite in there. Get locksmith grade graphite at the hardware store. Make sure it's the powdered kind, not liquid.
My ZX3 did it ONCE but when I got the key out after a lot of work, it didn't do it again. I graphited the cylinder though, and it's never happened to me since.
Just graphite powder. when you puff it into the keyhole and move the key in and out, it will get into the inner workings of the lock. Graphite will then lubricate (in a dry way, not a messy, dust-attracting way like oil) the lock and maybe help it turn. If that doesn't work though, you have to drill the lock to replace it which can be dangerous (you might end up ruining and having to replace the whole steering column), or have the dealer do it.