The type or namespace name 'UpdatePanel' does not exist in the namespace

Hi all,
Out of no where, I have began getting this error 
The type or namespace name 'UpdatePanel' does not exist in the namespace 'System.Web.UI' (are you missing an assembly reference? The UpdatePanel has been working just fine, then this came for no reason that I can identify
1. I have tried adding an UpdatePanel, cleaning, rebuilding, and removing that UpdatePanel as suggested here.
I don't know where else to turn as this error is so random.
thanks for any insight.
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