Yep, the router is configured with a Wifi Password. This password was set when the router was 1st configured.
Cabling up a PC to the router and hitting 192.168.1.1 will bring up the admin control for the router. Again, this router must have had the admin id/pw changed during the initial config.
If you don't have any of these items, or have forgotten them, then the only thing you can do is reset the router to factory defaults and start over. If you reset, you will lose ALL EXISTING CONFIGS. So make sure you know what you are doing and what is required to get back online. Is the modem bridging a conenection or does it make the PPPoE auth for you? If the router is doing PPPoE auth, make sure you have the id/pw. You reset to defaults with a paper clip inserted into the reset hole while powering on the unit (hold it in for 30 secs).
When you reset, you WILL be offline so make sure you have all the internet resources you need before trying it, including Linksys tech supp # and your ISP tech supp #.