How come when I play minecraft, it closes automatically?

I have minecraft on my computer and when I'm playing it the game closes automatically. I have been playing minecraft on my computer for a while until I got mad and decided to get minecraft for my xbox 360. The only thing is I want to get mods but I won't be able to use them on my computer because it keeps closing. I also can't get mods on minecraft xbox 360 edition (Not Really Sure) and I'm really upset about this. I'm thinking it's my computer because it's like 5 years old but it'spretty good so far.

Please help me it would mean a lot if you could help me with this problem. I really want to play mods like pixelmon on minecraft but I can't.

P.S. This works on both singeplayer mode and multiplayer mode.

If you are not running a minecraft w/ forge installed, it will try to run the mods and have nothing to run them on. Try taking the mods out of the mod folder and starting minecraft, see if that works. If that works, install forge for minecraft, and you can put the mods back in to the mod folder. Also check to see if your mods are in the right folder. And, make sure they are compatible with your version of minecraft. IF none of that works, just try re-installing.