My Hp slimline desktop is getting a 'no signal'

HP Pavilion Slimline S3400f Desktop.
dont worry ALL PC end up like this,, all do.. sooner or later..
NO OS stated, XP"? never post without OS stated, or answer get 10x more harder, See? make it easy for the helper and get more help.....
no Monitor stated. (maker, CRT, LCD what?)
The monitor is not fibbing to you BTW, the PC did go dead or to sleep. seems its monitor monitor of some kind with DVI.

if the monitor works on other PCs then the monitor is not bad.
i Bet it works (old ones 2002, have a hidden deep internal sleep mode. and is not yours IM sure now) DVI IS THE ONLY CLUE THERE.
So PC is dead. this PC came with only VISTA new,
is it still VISTA?????????
HP has support on it still
here (carbon dated by me 2008, IIRC)
https://support.hp.com/us-en/drivers/selfservice/hp-pavilion-slimline-s3400-desktop-pc-series/3688715/model/3688723

this year,? get all those files and books there backed up
when HP support ends after 10 year rule, (be savvy?)


what keyboard USB or wireless keyboard mouse
use wired USB keyboards if wireless the battery is dead. easy no?

now in VISTA, go to Control panel, click power
turn off all sleep modes and all hibernate modes
the turn off screen savers, if turned on , disktop . RC right click
in display settings or personalize, turn off , need login to come out of sleep or screen saver.
try a real keyboard, that has USB cable yet? be first thing to do.
always keep a real keyboard, handy,,,
batteries fail. in all wireless devices like this (inside them)

luck you desktops have lies DIM fails. there are many.

http://www.pcdied.com/sleep-or-dim.html

one other way , and common is PC overheated.
all do that.
ever one not cleaned out every year,.

#1 all time cause of overheat and PC shuts down.

http://www.pcdied.com/overheated.html

Try another monitor that you know works. If that doesn't work it may be your video card or chip.

Check the video cable connectors, remove the video connectors and reconnect them again.
Check the monitor input settings make sure the input is set to VGA and not to DVI.
Try a different monitor.

The most common causes are:

1. Loose video cable on either monitor or back of desktop
2. Bad monitor (only way to test monitor is hook it up to another machine or hook up a known working monitor to your existing machine.
3. Bad video card - this is less likely since most video cards are on board but if you have a different card, check to ensure it's seated properly on the motherboard.

from user manual troubleshooting section
(When connecting D-Sub cable) Check the signal cable connection. (When connecting DVI cable) If you still see an error message on the screen when the monitor is connected properly, check to see if the monitor status is set to analog. Press Source button to have the monitor double-check the input signal source.

Source button Activates a highlighted menu item.
/ Push the 'SOURCE', then selects the video signal while the OSD is off. (When the source button is pressed to change the input mode, a message appears in the center of the screen displaying the current mode -- analog or digital input signal.) Note : If you select the digital mode, you must connect your monitor to the graphic card's port using the DVI cable.
https://images10.newegg.com/User-Manual/User_Manual_24-001-063.pdf ninth page in user manual
source button fourth from the left when facing monitor. looks like a box with an arrow pointing left.

Have you tried plugging the monitor to a different computer? This will tell you if the monitor is bad if it works correctly in another computer then most likely your video card has gone bad and needs to be replaced.

If you saying "doesn't always boot", most common would be motherboard...
If your motherboard still beeps, then probably some other components has the problem like RAM, GPU card etc., depends on your motherboard.

Hope that give an idea...

it sounds as if the RAM is not being registered during boot. remove the RAM module, clean the contacts with a piece of paper & replace.

I take it that you are seeing the message "no signal" on the monitor. If this is true, then I would disconnect all the devices inside of the computer case except the CPU and memory. See if you get any video. If you do, then reconnect the devices (hard drive, CD, etc) until you find the one giving you a problem.
If you have done all this and still no video, I would suspect one of two things: Either the power supply or the motherboard.

1. after pressing poer button repetedly press f 8 go to safe mode enable vga mode.
if that does not work
2. get a pcie, pci, or agp video less than $100.00 that should fix it unless the mobo is ging bad itself.