Learn English – “last week” versus “over last week”

There are two differences between the sentences.

A number of reactivations were performed over the last week. [As snailplane suggests in the comment, a native speaker would use the.]

This sentence means that the reactivations being discussed occurred during the last seven days and probably at several different points (at least two) .

A number of reactivations were performed last week.

This sentence means that the reactivations occurred at some time during the last calendar week (which may or may not be the last seven days). In US usage, the phrase last week, (without the) almost always means last calendar week. There is no suggestion about whether they all occurred at the same time or were spaced out in time.

The sentences differ in the exact period of time being discussed and the spacing of the events. There are many circumstances when either could be used, but other circumstances when only one or the other would be appropriate.