how do I find my photo library in time machine

Go to you existing Photos library on your boot drive and then enter Time Machine. Go back in time to the one you want to restore. If you don't want to overwrite your current library Control (right) - click on the library in Time Machine and select "Restore Library XXX to…" and then select a different folder to restore the library to. You will need to have enough free space on your boot drive to hold the restored library.