|What was the situation before this? |
Was the computer working fine for you previously?
Were you fiddling with any components or connections inside the case just before your symptoms happened?
Did this happen after you attempted to flash the bios?
Were you getting weird symptoms before the computer stopped booting, such as spontaneous black screen reboots, or the computer Restarting when you had selected Shut Down in Windows??
Is the clear cmos jumper in the correct position (Normal, or similar)?
Whether your computer starts up automatically after the AC power has been disconnected and then is reconnected to it can depend on a setting in the bios Setup - usually that feature is disabled or off by default, but it can be set so the computer starts up automatically when the AC power to the PS is connected.
"desk top model rxa842"
"desktop vaio model pca-rx850"
Which is it?
"checked ram checked fine "
How did you determine that?
Did you run a memory diagnostics test on it?
In some cases any single ram module tests fine when installed by itself in any slot, but two or more modules may not be compatible with each other and memory errors occur when more than one is installed. If the number or errors is relatively small, that may only be evident when you run a memory test.
Ram that works in another mboard won't necessary work in yours - if it does work fine in another mboard that proves there's nothing wrong with it, but that doesn't necessarily mean it will work in yours.
On the other hand, ram that did work fine in your mboard previously should still work fine in your mboard - it's extremely rare for ram that worked fine before in your mboard to spontaneously go bad or to be damaged by some event, unless the user damaged it by doing something dumb. Even if it did go bad, if you have more than one module, it's even more unlikely more than one would be bad at any one time.
something dumb =
- ATX mboards are always powered in some places, including some contacts in the ram slots, as long as the ATX PS is connected to the mboard and the PS is receiving live AC power (or on laptops, as long as the main battery is installed) , even when the computer is not running. You should always unplug the PS, or switch off the AC power to it (or remove the laptop's main battery as well as the AC adapter) , when you fiddle with any components or connections inside the case (or the ram and hard drive, etc., that can be accessed from the outside on a laptop), otherwise you can easily damage something.
- if you ever install a ram module in a slot backwards and then power the PS/mboard and try to boot the computer , the ram slot the backwards module is in is instantly fried, and so is the backwards ram module, neither will ever work again, and even if you clean up the melted plastic and carbon from the damaged ram slot, the mboard may never boot again when there is working ram in other undamaged ram slots.
If in doubt about the ram, remove all the ram (when the PS cord is unplugged or the AC power to it has been switched off) and with the known good power supply connected, try booting - if nothing else is wrong, you will get no video but you will hear a beep pattern that indicates no ram is installed or a ram problem.
Usually when you have a ram problem, either one or more modules is incomptible with your system, or the ram has a poor connection.
A common thing that can happen with ram, even ram that worked fine previously, is the ram has, or has developed, a poor connection in it's slot(s).
This usually happens a long time after the ram was installed, but it can happen with new ram, or after moving the computer case from one place to another, and I've had even new modules that needed to have their contacts cleaned.
See response 2 in this - try cleaning the contacts on the ram modules, and making sure the modules are properly seated:
Depending on the mboard, you may need to connect an additional power connector from the PS to the mboard other than the main 20 or 24 "pin" one - if that applies did you connect that?
If the main socket on the mboard is 24 "pin", usually using a 20 "pin" connector from the PS alone does not allow the mboard to boot.
Some video cards in a slot require an extra power connection from the PS - if that applies did you connect that?