I have an older Sony Vaio PGC-K23 that had 256 x2 installed. I removed them so I could replace them with 512 and then reinstalled the 256s when I wasnt able to get the larger ones tonight. They were out for only a few minutes and I was careful with them. I am getting the two lights on for maybe a half a second and then nothing. its like a car that just wont start. It tries then nothing. i am not getting anything on the screen.

Any ideas would be appreciated.

Try reseating them. Take both the sticks completely out of the computer and stick them back in. The copper contacts on the sticks should be barely visible when the sticks are in place. You should hear a pretty loud snap when they go in properly and should be oriented parallel to the motherboard.

Try installing only one stick at a time. If one stick doesn't work, then remove it and try the other. Generally, one should be able to at least get to POST (Power-On-Self-Test) with just one stick installed.

Make sure the stick(s) is oriented the correct way as well. In some connectors it's actually fairly easy to install them backwards. However, if powered up with a stick inserted backwards, it's possible the memory and/or mobo was damaged.

For the same reason, it's sometimes very easy to not insert the stick(s) all the way into the connector(s), or install one a bit crooked. Again, it's possible damage has already been done.

ESD (Electro-Static-Discharge) also could have damaged your memory and/or mobo if appropriate procedures weren't followed.

Did you change anything in your BIOS? You may have to manually do a clear CMOS on
your mother board. Check your users manual on how to do this.