It seems to be a common problem that your Network and Volume icons may disappear from the system tray. If this happens, it may be a quick fix by going to the properties of the taskbar and checking off each one on the Notification Area tab. If however, these options are greyed out, then the problem may be the icon “History”. To resolve this, delete the following keys in the registry then restart the Explorer.exe process.
HKEY_CURRENT_USERSoftwareClassesLocal SettingsSoftwareMicrosoftWindowsCurrentVersion TrayNotifyIconStreams
HKEY_CURRENT_USERSoftwareClassesLocal SettingsSoftwareMicrosoftWindowsCurrentVersion TrayNotifyPastIconsStream