I see pretty much all the answers recommend deleting the lock. I don't recommend doing that as a first measure; maybe if there is no alternative. The lock is placed when an apt process is running, and is removed when the process completes. If there is a lock with no apparent process running, this may mean the process got stuck for some reason.
If you try
ps aux | grep apt
that will catch processes containing the word apt, at least. If you see an apt-get process or an aptitude process that looks stuck, you can try
kill processnumber
and if that doesn't work try
kill -9 processnumber
This should kill the process and may remove the lock. Killing an apt or aptitude process is harmless unless it is actually in the middle of package installation. In any case, if the process got stuck, you probably don't have a choice but to kill it.
Killing a dpkg process directly, if present, is not a good idea, because if dpkg is active, it is probably manipulating the package database, and killing it may leave the package database in an inconsistent state; i.e. corrupted.
Killing an apt-get or aptitude process is in general much safer.