Hollaz..

I've just upgraded the new version of iOS 4.3 on my iPad 16GB (3G + WIFI). However, I've found that all my previous apps (games, fitness and etc..) which I've install before cannot be launch anymore. Cydia software also MISSING.

Is there any possible to downgrade from iOS 4.3 to 4.2.1?

Needs some help here..

Got Cydia means it jailbreak.
No jailbreak discussion here

to answer your question. No you cant downgrade the OS.
Cheers

unless you ever save you shsh blob previously when you running your 4.2.1...then you can restore, or else, nope..

Why not? (just out of curiousity). JB is not considered illegal in the US. It does invalidate the unit's warranty in case something happens.

Reason

Even it's considered legal in US but most of the JB topic here will lead to piracy at the end, therefore it's the rules of Lowyat.net that prohibited any JB, piracy and warez discussion.