Error Creating Controls - Specified Cast is Not Valid

Question

When I close my c# solution in .NET and reopen it, instead of having my controls on my webform i have a bunch of placeholders that say "Error Creating Controls - (control ID). Specified Cast is Not Valid."  Can some please tell me why this occurs? 

Additionally, when I try to build my solution, I get an error that says,
"Could not copy temporary files to the output directory."
"The file 'bin\Project1.dll'  and 'bin\Project1.pdb' cannot be copied to the run directory.  The process cann access the file because it is being used by another process."

I have made sure the Copy Local Property is set to True for my references.  Besides that I don't know what the problem is.  If anyone can help me with these issues I would appreciate it.  Thanks in advance.