Magento 2 Get customer ID from session in a block class

It's working copy. You can compare with your block class. Here I use Form as block class

namespace Vendor\Module\Block;


class Form extends \Magento\Framework\View\Element\Template
{
    protected $customerSession;

    /**
     * Construct
     *
     * @param \Magento\Framework\View\Element\Template\Context $context
     * @param \Magento\Customer\Model\Session $customerSession
     * @param array $data
     */
    public function __construct(
        \Magento\Framework\View\Element\Template\Context $context,
        \Magento\Customer\Model\Session $customerSession,
        array $data = []
    ) {
        parent::__construct($context, $data);

        $this->customerSession = $customerSession;
    }

    public function _prepareLayout()
    {

        var_dump($this->customerSession->getCustomer()->getId());
        exit();
        return parent::_prepareLayout();
    }
}

When You define block which use session You have to disable cache for it.

 <block class="Vendor\Module\Block\Index" name="Name"
 template="Vendor_Module::template/path.phtml" cacheable="false">
 </block>

You need to inject \Magento\Customer\Model\Session $customerSession, class to get customer ID from customer session.

protected $_customerSession;

public function __construct(
    ...
    \Magento\Customer\Model\Session $customerSession,
    ...
) {
    ...
    $this->_customerSession = $customerSession;
    ...
}

public function getCustomer()
{
    echo $this->_customerSession->getCustomer()->getId(); //Print current customer ID

    $customerData = $this->_customerSession->getCustomer(); 
    print_r($customerData->getData()); //Print current Customer Data
}

NOTE: You only get customer id if customer logged in and customer session initialized