Apex Debugger Troubleshooting | Force.com IDE Developer Guide

Relaunch Your Debug Configuration
If your code isn’t stopping on breakpoints, variables aren’t displaying in the Variables pane, or you don’t see turning yellow gears in the Debug pane, relaunch your debug configuration.

Kill an Orphaned Session
If Eclipse crashes or is shut down before you end your debugging session, your session is orphaned. You can’t start a new session until you remove the orphan. But don’t panic! You can kill an orphaned session in Setup or with the Tooling API.

Change Your Session Timeout Preference
If you’re on a slow network, you can change the default timeout setting for your Debugger connection. This setting determines how long Eclipse waits for the Debugger to respond when you take an action, such as stepping through your code.

Report Drastic Issues to Customer Support
It’s unlikely that you’ll encounter drastic issues while working with the Apex Debugger. However, never say never. If you encounter performance issues or are unable to kill your orphaned sessions, contact Salesforce Customer Support for help. If necessary, Support can kill all debugging sessions for the instance on which your org is running. This process prevents you—and all orgs running on your instance—from immediately initiating new debugging sessions.