ubun2

I have a Dell Vostro 3450 i installed ubuntu and ran it a couple times perfectly, now when my computer boots and i select the operating system it always boots my windows 7 but when i boot ubuntu it boots normally and works fine sometimes but other times i select ubuntu and the screen just goes black and doesnt boot ubuntu.

I've upgraded my 9.04 to 10.04 last night and the install seem to go well but after the restart, all I see on my screen is blank after the startup logo.  Anyway to correct this?  I'm going to download and try starting the 10.04 by cd and see if this happens again.  If you no of an fix, please email me at sparkcad@gmail.com.
thanks in advance.