Got an error while using CRM 4.0

I was executing CRM 4.0 via web in my PC when randomly I got this error message (see below). I tried restarting the system and even the suggested restart of the PC Server didn’t resolve the error.
Any advice on how to resolve the error?

I am really new to CRM, so the provision of a simple step by step procedure on how to fix the problem will be greatly appreciated. Thanks.
Invalid Action
The selected action was not valid.
