Hi and Welcome. My Name is ***** ***** I look forward to assisting you today with your question.
You or someone has turned off the simple passcode option from Settings. When the simple passcode is turned off, it will ask for password ***** of 4 digit passcode. Don't you remember doing that ?
If you don't know it, then unfortunately there is NO way to retrieve that password ***** get back to your iPad without the password. Your only option is to force-restore the iPad to factory settings through iTunes by putting it to Recovery mode, which will erase everything from it. This is how Apple designed it to protect the information of their users.
But if you hadbacked up your iPad to iTunes on your computer or iCloud, then you can restore all your data back after restoring your iPad from iTunes/iCloud. You can also redownload all your past purchases for free using the same account.
Follow the steps below to restore your iPad by putting it to recovery Mode.
First download and install latest iTunes on your computer from here : https://www.apple.com/itunes/download/
Then connect the iPad to your computer with the USB Cable.
Open iTunes application on your computer.
On your iPad press and hold the Home button and the Power button at the same time until it shows a cable pointing to iTunes icon.
When done correctly, iTunes will show a message telling you that it has detected an iPad in recovery mode.
Click OK or Restore
Note : Put it to recovery mode while connected to computer. It will take a while to restore it.
Please let me know if that didn't work, so I will continue to help you until we fix the problem.
Remember, this is not my final answer. If my solutions didn't work please don't rate me bad or poor. Instead please reply back to me and I will continue to help you until we fix the problem or you are satisfied.