Drive A system error/ startup halt

Hello, everyone. My computer was working fine at 11:00 this morning. i went upstairs after i launched diablo2 (old i know) and when i came down it was beeping. 1 long then 2 short beeps. i shut it off and re-booted a few times then it and went to the first code screen (loaded all that or w/e) then on the second code screen the insert thing when u type just blinked.. i restarted again and went back to beeping. I opened it up and make sure everything was plugged in and not loose. I have no idea wat else to do. I have had this computer crash about 2 months ago and i put new in and i think new harddrive. key word *think*. Plz help me fast.