It has been observed that, after upgrading there are some common well known issue. Recently I upgraded my one of the store to Magento version 1.9.2.1.I face issue like, I can't able to login either back-end or front-end. So I come up with solution as below,
1) Manually remove the var/cache
2) Manually remove the var/session
3) Manually remove
the var/ locks
4) Adding form key to register.phtml
<input
type="hidden" name="form_key" value="<?php echo Mage::getSingleton('core/session')->getFormKey()
?>" />
The default register.phtml files, we can find at below location,
1) app/design/frontend/base/default/template/customer/form/register.phtml
2) app/design/frontend/base/default/template/persistent/customer/form/register.phtml
Then, refresh cache
and re-index solved my issue.