@Antoine553 remove generation and di folder from var folder, then check it.
i found the solution
<?php
namespace Mymodule\AdminPage\Block\Adminhtml;
use Magento\Backend\Block\Template;
class Contactslist extends \Magento\Backend\Block\Template
{
	
	 public function __construct(
         \Magento\Backend\Block\Template\Context $context,
         \Mymodule\AdminPage\Model\ContactFactory $contactFactory,
         \Magento\Framework\View\Result\PageFactory $resultPageFactory
     ) {
          parent::__construct($context);
          $this->contactFactory = $contactFactory;
          $this->resultPageFactory = $resultPageFactory;
     }
	
	public function mycustom(){
    	echo "test";
	}
	
	/*public function getAll(){
		$data = $this->_objectManager->create('Mymodule\AdminPage\Model\Contact')->getCollection();
		foreach ($data as $d )  {
		        echo $d->getToreplace();
		        echo $d->getReplaceby();
		    }
	}*/
	
	
}
					
				
			
			
				
			
			
				
			
			
			
			
			
			
		Issue was in your block file,
Original Class is
\Mymodule\AdminPage\Model\ContactFactory $contactFactory,
You have kept
\Mymodule\AdminPage\Model\Contact $contact,
So you have to keep Factory Object of your Model Class.
After Apply ModelFactory You must delete var/generation folder.