|Power everything down. Disconnect the power cable and hold the power button in for about 30 seconds.|
Reseat the battery (instructions on page 38 here ftp://ftp.dell.com/Manuals/all-products/esuprt_desktop/esuprt_dimension_desktops/dimension-e521_service%20manual_en-us.pdf just in case you need them.)
Power everything back up and see if that works. If it doesn't, replace the battery.