Under Tools -> Options -> Projects and Solutions -> Build and Run look for the drop down list "On Run, when build or deployment errors occur:". Change it back to "Do not launch" or "Prompt to launch". You probably inadvertently hit the "Do not ask me this again" check box in the "Prompt to launch" dialog.
Also, under "Projects and Solutions -> General" you can check the "Always show Error List if build finishes with errors" option.
-Peter
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