If you cannot remember the passcode, you will need to restore your device using the computer with which you last synced it. This allows you to reset your passcode and resync the data from the device (or restore from a backup).
If you restore on a different computer that was never synced with the device, you will be able to unlock the device for use and remove the passcode, but your data will not be present.
You may have to force iPad/iPod into Recovery Mode
No, i haven't passcode on my iPad. I need only slide to unlock. And slider has been working properly.
After i've slided, main screen with app icons has appeared properly. IBooks has been working properly, volabulary, safary, notes and other apps too.
But there was padlock icon instead of time at the center of status bar... (and other strange behavior - see my first post)
My guess (after that i asked the question) was so: there is opportunity to lock iPad partly. So i can for example use my apps, but can't change settings, turn off, connect to wi-fi and so on... But i couldn't find any information about it...