Running a heavily used 2009 27 iMac, with 16GB RAM, i7, and a Crucial MX100 SSD. The Mac just COMPLETELY locked up. Only the mouse would work, though the cursor wouldn't change appropriately dependent upon what was being hovered over on screen. Had a few Safari tabs open, Xcode, iBooks, Mail, and Activity Monitor. Never did this before. Running Yosemite. Had to hold down power button to turn off then turn back on; even Command + Option + Esc didnt work. Any ideas? Anyone else have this happen, if not even on Yosemite or on a Mac other than an iMac?