Resonators are powered by XM. Resonators are deployed with a full charge, but the XM store is consumed at a rate of approximately 10% per day. Attacks on Resonators by XMPs also reduce their XM charge. Resonators will self-destruct when the XM charge reaches 0. If all Resonators deployed on a Portal go to zero, the Portal will revert to Neutral status.
So to prevent your portals from decaying and their links/fields that they are associated with decaying it is important to keep a portal key handy and remotely recharge those portals.