If loss of a control element causes an increase in gene expression, what must be the normal role of that control element? propose a biological explanation for how the loss of such a control element could lead to an increase in gene expression.

The normal role of this control element can be that of a negative modulator or regulator

Explanation:

For example, the lactose operon in the bacteria Escherichia coli is negatively regulated. In lactose absence a represor protein is produced, which inserts in the DNA operator site blocking transcription of structural genes. if for example a mutation affects the synthesis of  this repressor, expression of structural genes will increase with no regulation.

A mutation in the DNA operator site that avoids insertion of the repressor, too.

A mutation in the gene that codes for the repressor, will also do the job