I think that your problem is not the sort of issue that could be easily resolved without looking at the data.
I don't think that having an overdrawn bank would cause a problem.
I have seen this happen when entering a module, but not the application as a whole
Sage data can get corrupted and that is why there is an error checking and backup options.
You can not get into the program to run error checking now, or to restore a previous backup.
What I am going to suggest is DANGEROUS, you should only do this if you are confident you have a good backup, if you want you can zip your data up and send it to me and I can fix it for you and send it back, but there would be a charge. I would estimate £25
http://www.makingithappen.co.uk/support.htm
You need to confirm that you are not using a username to log on, you just go straight in or enter a password and go in. It is at this point that the program stops.
You need to backup, you can't use the sage backup facility because you can't get into sage, so using windows explorer go to
C:\program files\sage\
(If you have upgraded or customised your installation this might be different)
There you should find a folder called ACCOUNTS this contains all the accounting program settings and data , copy the entire folder to your desktop or some other known place to keep safe as a backup.
Now go into the accounts directory
Here you will find a directory called TaskOpt go into that directory and delete the file called manager.xml
Try the program and see what happens.