Error when trying to add a custom web part to a page

Hi,

I developed a custom visual web part in Vs2012 and I have tested it against Sharepoint 2010 in my test environment. However, when I upload the web part to the solution gallery in prodcution and try to add it to a page, I receive the following message:

"Unable to load assembly group. The user assembly provider threw an exception while trying to provide user assemblies for the specified assembly group."

Can someone please tell me what I can do to fix this error and add my web part?

thanks,

SA

Sherazad