Is it clean? fan and vents? Unplug, remove battery, leave battery out, plug in, reboot. If that fails, Re Boot and try a System restore/recovery to a date before this issue. OR, Boot to Safe Mode, use this procedure to select Normal Boot:
http://www.netsquirrel.com/msconfig/msconfig_win7.html When/If you get a Normal Boot, Open Control Panel, Power Options, select High Performance, select Change Plan settings, set plugged in both to Never, set advanced setting to your preference. Use this procedure:
http://windows.about.com/od/customizingwindows/ss/set_power_options_all.htm Always shut down and unplug your laptop when not in use, remove battery when you use on AC, your Laptop/components will last longer, and will not be subject to possible power surge damage and the battery, IF it malfunctions will not cause a fire hazard.