There are few possibilities based on how you are trying to debug.
1. If you are attaching the aspnet_wp.exe manually from Debug menu, make sure that you are building all projects in that solution and at last build the web application/website project.
2. Sometimes, that doesn't solve the problem so you may try closing the IDE and open your solution again.
3. If you hitting F5, sometimes this also doesn't detect the break points, so the again before hitting the F5, ensure that you can build all the related projects and then hit F5.
4. Check that the OnlClick event name is same as the event name you have written in the .aspx.cs page (exact name with same upper-lower case).
Please let me know if above solve your problem.
Thank you.
Regards,
Sheo Narayan
http://www.dotnetfunda.com
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