Suspension alert is sent in case your monitor fails (sends NOKs) continuously with every check for 24 hours.
In this case, your monitor is suspended, meaning that Monitis stops monitoring for it. You can activate your monitor again at any time after.
To activate your monitor, go to the Monitors menu and select Monitor List.
The List of Monitors window will open.
Find your monitor in the list, and click on the rightmost icon next to the monitor name to open the context menu.
Click Activate to activate your suspended monitor.