What happened to the old power cord? If it was trashed by a power outage, short circuit or other such electrical issues, your Wii may of sustained damage as well.
Three months of inactivity certainly won't kill a Wii, but sitting around for a good while may of gotten the disc drive dusty. If certain parts in the drive are too dirty, it may fail to read discs until properly cleaned.
Do you know if the Wii had any physical trauma while it was sitting around unused? The disc drive's lens can be knocked out of place with a decent bit of blunt force, such as the Wii falling and hitting the floor.