Added more RAM, now CPU won’t start

I have an Intel Celeron 1.4 Gig computer, running XP,
with only 128MB RAM. Computer was a little sluggish
and so I went out and got more RAM. 512MB of RAM
today, that’s the max amount of RAM for the CPU. I
took out the 128 and put in the 512. I went to start
the computer and it starts normal for about 2 secs.
and then nothing. I hear one beep and normal lights
on front of CPU at first. Then the CPU is just “on” with
nothing happening. The back fan is on if that means
anything.

Then I removed the new RAM and put the old RAM in.
The same thing happens and the computer won’t
start. The computer worked fine up until 40 mins ago.
Any ideas would be extremely apprciated!!!!! Thanks!