There is not pre-defined keyboard shortcut in SharpDevelop for debug break. It is available from the Debug menu by selecting Debug - Break. You can also press the pause button in the main toolbar.
If you want a keyboard shortcut then you would need to edit the ICSharpCode.SharpDevelop.addin file and add a shortcut to the Break menu. Below shows the Break menu item, without a keyboard shortcut, and the Continue menu item which has F5 as its shortcut.
<Condition name="DebuggerSupports" debuggersupports = "ExecutionControl">
<MenuItem id = "ExecutionControlSeparator" type = "Separator" />
<Condition name="IsProcessRunning" isprocessrunning = "True" isdebugging = "True" action = "Disable">
<MenuItem id = "Break"
label = "${res:XML.MainMenu.DebugMenu.Break}"
icon = "Icons.16x16.Debug.Break"
class = "ICSharpCode.SharpDevelop.Project.Commands.BreakDebuggingCommand"/>
</Condition>
<Condition name="IsProcessRunning" isprocessrunning = "False" isdebugging = "True" action = "Disable">
<MenuItem id = "Continue"
label = "${res:XML.MainMenu.DebugMenu.Continue}"
icon = "Icons.16x16.Debug.Continue"
shortcut = "F5"
class = "ICSharpCode.SharpDevelop.Project.Commands.ContinueDebuggingCommand"/>
</Condition>
</Condition>
So you can add a new shortcut attribute for the Break menu item if you want to by editing the ICSharpCode.SharpDevelop.addin file.