not able to do transaction

Hi,
 
I am using authorize.net in my web application of asp.net. From last 1 year every thing was working correct. but now I am getting followsing error
"The underlying connection was closed: could not establish trust relationship for the SSL/TLS secure channel "
 why this error is comming and how to resolve it. I have checked ssl setting all are correct,
since i am using two payment method SIM and ARB. SIM is working correct but ARB is giving problem.
I have googled but not able to find any correct solution.
 
Thanks in advance
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