Remove the Umbraco.Forms.* files from the bin and also the 

<add name="ContourFormBuilderHttpModule" type="Umbraco.Forms.CodeFirst.ContourFormBuilderHttpModule, Umbraco.Forms.CodeFirst" />

lines from the web.config 

that should bring the site back, is this on a 6.0.2 site?

Just tested on Umbraco 6.0.2 and it worked fine, maybe you are installing on v4 of Umbraco?

In that case you'll need Contour 3.0.6, you can just grab the upgrade archive and deploy that on your site (should also fix the problem)

The website is still down.

I've removed Umbraco.Forms.* files from bin folder

There is nothing in the web.config that contains 'contour'

How do I get the exact version number when I can't get in backoffice ?

Check web.config for umbracoConfigurationStatus what does that say?

Shit: 4.9.0 :(

Just a sec I'll also try with that version and see what actions are needed

Hmm don't get the YSOD 

Is the System.Web.WebPages.Razor.dll in your bin dir?

Maybe try restoring the bin from a fresh 4.9 archive

Yse, the System.Web.WebPages.Razor.dll is in the bin dir

Ok also remove System.Web.Mvc.dll that should do the trick

Thanx a lot.
This got my heart racing for while :) 

Doh ... now I get this one.

Could not load file or assembly 'Umbraco.Forms.CodeFirst' or one of its dependencies.

I think I'll try with a clean BIN folder from 4.9.0

check your web.config for the contour entries, 

so remove 

<add name="ContourFormBuilderHttpModule" type="Umbraco.Forms.CodeFirst.ContourFormBuilderHttpModule, Umbraco.Forms.CodeFirst" /> 

should be in there twice once that is gone it should be back

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