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Remove power, remove the battery and anything else that is connected to the laptop. Let it sit for a few minutes, then press and hold the power button for 20 seconds. Then, leave the battery out, but insert the external power and try to power the laptop on. If that works, reinsert the battery and you should be good.
Let me know how it comes out.
I removed all sources of power, let it sit for 10 minutes and did as you suggested. The issues still exists.
With a Dell Latitude, there should be a battery charge indicator on the battery. Does it show as fully charged or empty?
If it shows as fully charged, try removing the AC power and just using the battery to power the system.
If it is empty, when you insert the battery and AC power, does the Charging Battery indicater light up on the laptop? (Probably just below the LCD screen or on the front panel.) If you know how to use a Multimeter, test the output on the AC Power adapter connector, where it plugs into the the laptop and confirm that it is outputting what the transformer shows it should output.
Also, you may try reseating the HD.
If none of this works, is the laptop still under warranty?
The battery charge indicator was on earlier this morning as it brought the battery up to full charge. The battery meter shows it to be full. Multimeter shows power adapter to up to spec on output. Results are same whether booting from AC or battery only. laptop is not under warranty.
When a system boots, it runs through a POST (Power On Self-Test). This laptop isn't getting to that point which indicates something is majorly wrong.
First, I'd try by resetting the BIOS (Basic Input Output System) which will give the system as fresh of a start as possible. Note that, if you do this wrong, you could cause severe damage to your comptuer. Follow the steps at http://support.dell.com/support/edocs/systems/latd620/en/UG/parts.htm#wp1106142. Once you remove the battery, leave it out for 2 minutes, then reverse the steps to insert it back in and close the laptop up. Take careful note to follow every step and adhere to every warning/notice in the instructions. Once you've done this, try booting with just AC power.
If resetting the BIOS doesn't work, the laptop needs to be diagnosed by a technician that can put their hands on it. Note that this won't be overly cheap and the repairs will probably reach into the realm of several hundred dollars. With that in mind, you might prefer to get a new laptop and scrap this one.
Let me know how this goes.
If you can boot it, try the steps detailed in the following URL: http://support.dell.com/support/edocs/systems/latd620/en/UG/trouble.htm#wp1305119.
If you can't get it to stay on, you might try removing the HD (Since its the last thing that was put in) and/or trying different Memory configurations to see if you can get it to boot and stay on for any time.
If you can't get it to even stay on long enough to get to run the tests w/out the HD in and with different memory configurations, it probably needs to be seen by a technician or you need a new laptop.
After reading the last post, I think I might have been a bit unclear. Once the system boots unsucessfully, its probable that you'll have to reseat the BIOS battery each time you want to try a different configuration.
On needing to replace the battery, I doubt it as the problem appears that it is keeping the BIOS config instead of loosing it. A dead battery should cause the config to be lost. If it is easy to get a new battery, its definitely worth a try.