The most common reason for Perfmon not working is corrupted or outdated drivers. To fix this issue, disable antivirus, update the operating system, check for corrupt system files, or perform a system restore. Another solution is to open the registry editor (regedit) and navigate to HKEY_LOCAL_MACHINESYSTEMCurrentControlSetServicesPerfProcPerformance. Search for "Disable Performance" and make sure it is set to 0.