It helps if you give the exact model of your laptop and the "Service Tag" (but not the serial number), but in general, a blinking red battery light means that the charge is low. If the light blinks when the charger is connected, it usually means that something is wrong with the battery or charger.
Other useful information:
What version of Windows is installed
Is the battery a "genuine" HP part or from an online discount supplier
Does the computer work properly, with no blinking lights, if the battery is removed and the charger connected
If your laptop is 2006 or newer (or 2005 and listed here), download, install, and run the HP Battery Check.