Ok tried to run an upgrade on a production site from 4.5.5 to 4.6.2

I made the web.config changes and the whole shabang but when i navigate to the site I get the yellow screen of death with the following error:

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.ArgumentNullException: Value cannot be null.
Parameter name: type

Please advise!!

I keep getting this over and over again even when i delete and restore the database and refresh the code.

Is there another way to upgrade my site other than this mechanism?  I thought of redoing it from scratch but we've got over 10,000 users right now and i'd lose all of that info which isnt' an option for us.  The site is quite sizeable as well.  PLEASE, PLEASE HELP.  I don't know what else to do!!!!

One thing I noticed is that in my database in the version table are three new entries for 4.6.0, 4.6.1, and 4.6.2 but when I login in host settings it still says that the site is 4.5.5?  Could it have run partially and now the database is halfway done?  If so how do i correc this?  I can't rollback to an older database because people have been on the site since then registering or doing other business. 

Please advise ... I need help!

Thanks Ian for your response.

I think I know what the problem was.  I finally got the upgrade to run successfully.  Here is what I did (in case anyone else experiences this)

In our web.config we have a lot of third party http mods.  I was putting all that extra info in and then trying to run the upgrade resulting in the error.  I tried it again this time only changing the connection strings, machine key, install date, and object qualifier (the bare minimum).

The upgrade ran fine and I put the other stuff in afterwards.  Still working fine.

Also I deleted the 4.6 rows from the version table in the db before  trying the upgrade again.  Worked like a charm.  Hallelujah!