Thank you for all feed-back.

Actually this happened after the server was shut-down because of a power failure and the database entered to "In Recovery" mode after a long time the database was on-line and everything was OK.

Last friday I ran the command again and the problem has gone...

Now I would like to know what I should to do keep my transaction log tiny because after I ran the command above I've shrinked the log file from about 20GB to only 15 MB (yes... Megabytes). I have my database mirroring to another server and I really need to have the transaction log more small because with this size I have to wait more than up to three hour to bring on-line a database. What maintenance I should do in the databases to keep the transition from a Mirror to on-line more smooth and quick?