Simsy - there is an item here on replacing a "power light" LED which should help as to how to proceed - pay attention to No. 7 in list - click here
Item No 7 - Take the bulb and the wires to a local computer parts store. Ask the clerk to help you find an exact replacement. In most cases "close enough" is not good enough: LED bulbs that are too large will not fit in the front panel correctly. If the store does not have a good replacement, try searching for the part on line.
Read more: How to Change the LED Light for the Power Button on Your PC | eHow.com click here