You really only have one choice when your Linksys device's default password or default username has been changed and you don't know what they are: reset the device to factory defaults.
For most Linksys devices, a factory reset is as easy as pressing and holding the tiny Reset button for at least 10 seconds (30 seconds on older models) and then unplugging and plugging back in the device.
If that doesn't work, check your router or other device's manual where you'll find specific advice for that Linksys model. You can find your device's manual in PDF format from Linksys Support.
A factory reset will also return the IP address that's configured for your Linksys device back to 192.168.1.1, or whatever the default may be, but doing so could cause problems.
If your Linksys router's IP address is all you're after, please know that it's also almost always the same as the default gateway that's configured for your computers and other devices that connect to it.