The value is not terribly critical, but it's normally selected based on the switching frequency of the power supply. All it does is prevent electrical noise from leaking out of the power supply back into the wiring and interfering with radio reception. You can run the computer without it on a temporary basis, but it's best to replace it with something when possible.

I've seen a number of them fail. Usually smokey and smelly, but the X2 caps are rated as such in that they are highly resistant to actually catching fire.