Task bar disappears

I have Win 7 x64 and I left my i7 920/GTS 250 machine running last night and I woke to an invisable/nonexistant taskbar. I am hoping someone knows from experience why this is. It has happened repeatedly when the machine is left running for long hours untouched. Thanks guys!

This website is not affiliated, owned, or endorsed by Microsoft Corporation. It is a member of the Microsoft Partner Program.