No display on moniter but cpu is running

those are blink codes, learn to count them.
if you pulled the CPU i bet you that you used the wrong heat sink compound.
The CPU shuts off all by itself if too hot. your job is to get the fan working and the heat sink working and that thermal compound right.
hp.com
as the blink codes we are not HP here, ever

https://support.hp.com/us-en/document/c02222922

try disconnecting all hard and optical drives. Try to power on. If it works one is probably faulty so reconnect one at a time to find the culprit.
Check the power supply also.

I have lg flatron l1730s lcd black color monitor. I have problem with this. Power not come. some time when i connect it power LED blinking once then no display.

When you power up then after 12 seconds check the num lock led status on key board press num lock key then if num lock led is on and off then your cpu is working fine.
For that make sure vga adapter from monitor is connecxted to cpu is properly.Then when you power up the computer press F8 key then you can see some menu enter in "Enable vga mode " then go to display property and change screen resolution to 1024*768 then apply and check.
But if num lock led is constant then then you need to open cpu cabinet cover.
Then reconnect ram memory .Try it in different ram slot.Make sure motherboard is not ground with cabinet.REmove pci or other card if any.
Then check.
Pull cmos cell from motherboard for 10 minutes then connect again.
Also reconnect processor and it's fan then check.
Let me know if you need further assistance.
Thanks for using FixYa.i

Check your num lock key status press it on and off if hum lock led is on and off when u pressing then your cpu working fine. so check connectiopn of vga cable cpu to monitor.
Also some times display screeen resolution issue so when u power up press F8 key then enter in enable vga mode then you can chang escreen resolution to 1024*768.
but if num lock key is not on and offf then open the cabinet of cpu then find cmos cell on mother bord and pull it for 14 minutes then check .

low power consubtion first on ur moniter then cpu

remove the battery and try to start the machine om AC power only

regards

1. cpu problem. why because of there is no signal output from cpu to monitor. the led blinking means there is no vertical, horizontal signals from cpu to monitor.

2. your monitor connected to another system.

3. neck board dry points.

4. cheak the display cable. 13&14 pins are horizontal, vertical signals.1, 2, 3 pins , R G B , 6 ,7, 8 pins R G B ground ( coaxiler cable used , meshed wire is ground )

if this is a built sustem out of the box with old monitor try another monitor oddly enough some arent compatible, you can check by looking at the prong patern.

Have you upgraded the memory?