mongod not logging after log rotate - mongodb

Question

Originally, mongod is running and log to file shard1.log. And I run mongod again and failed.
In the shard1.log, there are only infomation about the failure of second command. The origin log is renamed to shard1.log.2015-02-01T07-41-46.
But the problem is the origin mongod not logging any more.
I check the opened file of origin mongod.
ls -l /proc/7980/fd | grep -v socket | grep log
l-wx------ 1 mongodb mongodb 64 2月 6 06:05 1 -> /data/mongodb/log/shard1.log.2015-02-01T07-41-46
The shard1.log.2015-02-01T07-41-46 stayed unchanged.
So how can I make the origin mongod log again?
Edit
here is my mongo config file
logpath=/data/mongodb/log/shard1.log
fork=true
dbpath=/data/mongodb/db/shard1
pidfilepath=/data/mongodb/pid/shard1.pid
shardsvr=true
replSet=shard1/10.0.0.1:27017
port=27017
bind_ip=10.0.0.2
the output of command db.serverCmdLineOpts
{
"argv" : [
"/data/mongodb/bin/mongod",
"-f",
"/data/mongodb/conf/shard1.conf"
],
"parsed" : {
"bind_ip" : "10.0.0.2",
"config" : "/data/mongodb/conf/shard1.conf",
"dbpath" : "/data/mongodb/db/shard1",
"fork" : "true",
"logpath" : "/data/mongodb/log/shard1.log",
"pidfilepath" : "/data/mongodb/pid/shard1.pid",
"port" : 27017,
"replSet" : "shard1/10.0.0.1:27017",
"shardsvr" : "true"
},
"ok" : 1
}