Did you replace the cap? Also, I would check the fluid levels, if it came off, you may have had some leak out of the system. I've had situations where this happened with low coolant levels, and that's never a good thing..
2011 Grabber Blue GT California Special, 3.73 Gears, 6-Speed Manual, Steeda CAI, Steeda 93 tune, aluminum driveshaft (The Driveshaft Shop Co.), MGW shifter, Roush axle-back exhaust, Roush Side Rocker Panel Splitters, Ford Racing BOSS 302 front splitter, sound-tube delete, painted plug covers, JLT oil separator. Shelby transmission scoop & powder-coated trunk brace & many more items...