If the task bar in that system is an XFce panel, you should open a terminal (you can probably do that by right-clicking the desktop, then choosing Accessories -> Terminal if it's XFce) and there enter the command xfce4-panel.
That should re-establish a lost panel if it got closed. Don't close the terminal, or you'll lose the panel again; simply log out and back on while saving the session (there might be a checkbox for that in the log out dialog). The terminal will open again but this time around you can close it and the panel will remain.
If that doesn't work it is possible you have somehow set the number of panels to 0. XFce has a general settings dialog, in which you can adjust settings for the panel, and you should re-set the number to 1. The standard panel should re-appear then.