Is the power button LED, Steady Blue, Blinking Blue, Steady Amber, Blinking Amber?
Are there any 'Beeps', or error messages, if there are, please post the exact text of the message?
Have you made any changes to the Vostro 400 recently?
The Vostro 400 Troubleshooter and Diagnostic Codes are HERE. <ADMIN NOTE: Broken link has been removed from this post by Dell>
one beep, steady blue light, steady dell screen. No recent changes to anything. Computer was a bit slow, so I figured a reboot was in order. It never came back to functional. Thanks Bev
Remove all the peripherals from the 400, except the mouse, keyboard and monitor and see if the system boots correctly.
No difference, turn off Vostro 400, remove power cord and hold power button in for about three minutes, reconnect power cord and power system on and see if this makes a difference.
No diference, for one 'Beep', according to the Vostro 400 Troubleshooter, indicates a BIOS checksum failure. Possible motherboard failure.
Try the following, if you are comfortable working around computers:
Note: The only 100% method of testing a PSU, is to install a know working power supply.
Unplug the cord from the power supply, hold the power button in for about 15/20 seconds, open the case, unplug the 24-pin power connector from the motherboard and jump the Green wire to one of the Black wires, reconnect the power cord and power the system, if power supply's fan and the hard drive run, then the PSU should be good.
Note: Do not remove any wires from the plug, use a small piece of wire or a paper clip as a jumper.
Power supply checks out, again remove the power cord, hold the power button in for several seconds to discharge the residue power, reconnect the 24-pin connector to the motherboard.
Remove all PCI cards from the system, take out the video card [if applicable], disconnect power & data cables from all drives.
Reconnect the power cord and power system on, see if you get a different LED indication on the power button, or 'Beeps'.
No difference, remove the memory from the system and try again.
No difference, then you are looking at a motherboard replacement & maybe a processor.
I just wanted to add to this thread for any future readers. We have a Vostro 400. Left the office one day it was fine. Came back the next morning... it was dead. Not one sound out of it. It wouldn't do ANYTHING. Green light on back of power supply was on. Yellow light on mother board was on, but absolutely no response. Talked with Dell which resulted in no help. I tried the mentioned green/black jump (which I also read elsewhere online) and the fans came on along with the hard drives spinning. I also had the power supply tested locally and they said it was fine. We moved on to the motherboard and processor - ordered both. Guess what... it WAS the power supply. Even after it supplying some power to fans and drives and having someone test it, it was still faulty. The box is now back to original parts except for the new power supply from a local big box electronics store.
Sidenote: Dell totally dropped the ball trying to get a motherboard to me, so we did get that elsewhere. A few calls to Dell resulted in phone reps not really knowing what to tell me about our problem followed by one hanging up on me before we even got rolling followed by the last one botching the order so it never shipped out to us. Not a good image-building experience and we've been very loyal to Dell for many years.