TV does not have good (any?) error checking in its Transaction Definition Editor and you can easily add data that can either corrupt the Txn Editor (or even the whole analyzer)... For instance, adding an XML snippet into a field breaks everything. The previous solution was to drop and rebuild the txn database.
An alternative solution is to use the DataUtil(.bat / .sh) found in <TVHOME>/bin.
% DataUtil.bat -clear_txndef
This will blow away all your definitions... So you will have to rebuild from scratch, but it is easier than rebuilding the db. You can also add the -schema <SCHEMA_NAME> option, but I'm not sure if that really limits the impact.
A good practice is to export your (working) txn definitions before starting a new one. You can use the DataUtil to export txn definitions to .xdm files.
% DataUtil.bat -export_txndef <out_file_name>.xdm
You can then import later if you have a problem.