i want to open my iclod ipad2 wifi

You need to type in the iCloud account that you used in Settings > iCloud on the iPad, and its password. The activation screen should show the first letter of the account that you used, and its provider, which should give you a clue as to your id.

If you can't remember your account then do you have any of its downloads on your computer's iTunes : Recovering a forgotten iTunes Store account name

Or try finding it via http://appleid.apple.com : Apple ID: How to find your Apple ID

If you can't remember or find your id then you will not be able to reactivate your iPad

If you are on the activation screen then you should see the first letter of the email address - you don't remember having an email address like that provider, and starting with that letter ?

If not then have you tried finding it via either of those two links (checking your library for downloads and going to http://appleid.apple.com) ?

You won't be able to use the iPad until you can remember or find your id.

No.

How can you know the password but not remember your Apple ID?

Then try to recover the ID at https://iforgot.apple.com/appleid

If that doesn't work, you will have to call Apple and speak to someone in account security.

If it is locked with an account that is not yours, then it's a brick. Only the owner of the account can release the lock.

Call them.

https://www.apple.com/support/contact/

What do you mean how can you speak with them? Look up the phone number using the link I posted, call them, then speak.

What is the problem with doing that?

Apple does not provide support via email.