How come my computer acts like it wants to boot but does not boot?

Q: I have a tower that when i turn it on acts like its trying to boot it gets power and the cd rom runs but thats it it doesnt boot up or anything i tried unplugging the primary hard drive and running off the secondary and nnothing it still does the same thing everything as far as i can tell is getting power and running could this be a motherboard issue or is something else going on that i may not be seeing