Pi lost power now I get file system warnings at boot.
I guess you're on Raspbian. Clearly, you should avoid power off your Pi without shutdown. You should always use the command :
sudo shutdown -h now
If you power off the Pi without shutdown and the Pi was doing something with the filesystem while your turned off the pi it can cause several and unknown crashes.
As the system tell you you can try to check for the file system integrity following these steps :
2) Since it's asking your to start a fsck type :
fsck.ext3 -y /dev/...
4) Answer the questions the program ask you.
In my opinion your FileSystem is damaged/corrupted but I'm not a pro linux user, wait for some other opinion maybe. If this is really the case the only solution I know (maybe there are some other I don't know) is to reinstall your OS...
Hope it will help you anyway.