This knowledge base article provides steps to troubleshoot and resolve "No recent performance data available. Is the system online?" messages for a Windows Agent element in the Global Scan tab. A similar article for WMI elements can be found in "No recent performance data available. Is the system online?" - WMI elements.
This message indicates that the Platform Performance Gatherer (PPG), a background service monitor responsible for gathering performance metrics, such as CPU, memory, disk, is unable to properly collect metrics. The most common reason the PPG fails for Agent elements is corrupted or unavailable Windows performance counters.
To verify if and which metrics the counters are failing for, run Poll Agent tool for Windows element.
The metrics where the counters are failing will likely contain an ERR string. To resolve try rebuilding the performance counters using the steps outlined in the article Rebuilding Windows Performance Counters.