MongoDB won't start after server crash

My Ubuntu computer had crashed, and when I restarted it MongoDB wasn't working. I tried the following commands, and got the following output: $ mongo Error: couldn't connect to server 127.0.0.1:27017 src/mongo/shell/mongo.js:91 exception: connect failed $ service mongodb status mongodb stop/waiting $ service mongodb restart stop: Unknown instance: start: Rejected send message, 1 matched rules; type="method_call", sender=":1.57" (uid=1000 pid=2227 comm="start mongodb ") interface="com.ubuntu.Upstart0_6.Job" member="Start" error name="(unset)" requested_reply="0" destination="com.ubuntu.Upstart" (uid=0 pid=1 comm="/sbin/init") $ tail /var/log/mongodb/mongodb.log [initandlisten] exception in initAndListen: 12596 old lock file, terminating dbexit: [initandlisten] shutdown: going to close listening sockets... [initandlisten] shutdown: going to flush diaglog... [initandlisten] shutdown: going to close sockets... [initandlisten] shutdown: waiting for fs preallocator... [initandlisten] shutdown: closing all files... [initandlisten] closeAllFiles() finished dbexit: really exiting now (Output reformatted to match website layout.) What happened? How can I fix it?