Hi installed the VM Invoice plugin and activated it and set to automatic mode and now I can't log in to the administrator back end and all I get is a blank page with the following message:

Not defined items block template for invoice

Please help!!

Hello,

Go to database and disable that plugin. You can do it in db table: #__plugins. Find there plugin by name: System - VMInvoice Autorun. Set published to 0.

Then you should have been able to log in to admin part of your website.
But there is still problem. Can you check you configuration in:
Invoice Configuration -> Invoice Template -> Items block

Are there filled both areas with template? Table Header and Table Row?
This message you get should be printed only if there are not both templates in database.