Did you change your router? Quite likely the issue is as described by Jay.

To find the new IP, you can either go to your routers admin console, and look at the list of active DHCP leases.  Did you give your RPi a name? If so, that name should show up here. If there are no names, OS should be mentioned. Find the ones that say Linux, and try to login in one by one, when you have the correct IP, login will succeed.