It's a capacitor. Solder it back on if you can. Running without it may make the PC unstable, or have out of spec voltages and ripple through to the cpu, which will eventually cause the cpu to fail. Get it fixed ASAP.
Its like if i pull, say, the air filter off my car. Sure it runs without no probs for now, but it is letting in debris and dust into the engine which will cause a problem sooner or later and permenant damage to the engine.