If Thunderbird crashes or there's a power cut while you have Thunderbird running, you may find that when you try to restart it again it either doesn't load or you get a warning message appear telling you that your profile is already being used. The reason for this is because when Thunderbird starts, it creates a lock file. The lock file stops anyone else opening your profile at the same time as you. Normally this doesn't cause any problems as this file is deleted when you close Thunderbird down but when the power is cut, programs don't have time to close down therefore the lock file remains.
One way to get around this is to find with profile you use, go into that directory and manually delete the file called parent.lock or you can run a little program that will do it for you. If you want to use the first option, open My Computer or Windows Explorer and type in %appdata%\thunderbird in the address bar. Open the file profiles.ini and look for the profile that includes the line Default=1. Note the profile name and location, this will normally be a sub-directory within the location where the profiles.ini file lives. E.g.
[Profile3] Name=hermes IsRelative=1 Path=Profiles/hermes Default=1
We can see that the path to the profile is in the sub-directory Profiles/hermes
Navigate to this directory and file the file called parent.lock and delete it. Now try starting Thunderbird again.
The second method does this work for you. From the Start Menu, select Programs and then Open DAMTP Programs. Run the program called Unlock Thunderbird. You'll see a black box appear on your screen and then a message will pop up to tell you the lock has been removed. Now try restarting Thunderbird.