I would like to share my experience with the Reset forgotten password page blank after installation of SECURITY PATCH SUPEE-6788.
I investigated and found that the latest customer layout
'customer_account_resetpassword' handle is replaced with
'customer_account_changeforgotten'.
So, if you have replaced the old layout handle with new one in magentoRoot/app/design/frontend/ur_package/ur_custome_theme/layout/
customer.xml file.
For Example:
<customer_account_resetpassword translate="label">
<label>Reset a Password</label>
<remove name="right"/>
<remove name="left"/>
<reference name="head">
<action method="setTitle" translate="title" module="customer">
<title>Reset a Password</title>
</action>
</reference>
<reference name="root">
<action method="setTemplate">
<template>page/1column.phtml</template>
</action>
<action method="setHeaderTitle" translate="title" module="customer">
<title>Reset a Password</title>
</action>
</reference>
<reference name="content">
<block type="customer/account_resetpassword" name="resetPassword" template="customer/form/resetforgottenpassword.phtml"/>
</reference>
</customer_account_resetpassword>
Replaced above code with code below...
<customer_account_changeforgotten translate="label">
<label>Reset a Password</label>
<remove name="right"/>
<remove name="left"/>
<reference name="head">
<action method="setTitle" translate="title" module="customer">
<title>Reset a Password</title>
</action>
</reference>
<reference name="root">
<action method="setTemplate">
<template>page/1column.phtml</template>
</action>
<action method="setHeaderTitle" translate="title" module="customer">
<title>Reset a Password</title>
</action>
</reference>
<reference name="content">
<block type="customer/account_resetpassword" name="resetPassword" template="customer/form/resetforgottenpassword.phtml"/>
</reference>
</customer_account_changeforgotten>
Your reset password page should work correctly now.
Please refer link below for more detail...
http://www.atwix.com/magento/security-patch-supee-6788-installation-issues/