Sometimes when an employee who has been good as gold suddenly becomes a lead brick, the reason can be as simple as burnout. Or it could be more complicated than that.
They may be having domestic problems, money worries or any number of other problems that are translating into poor performance at work. There are a number of ways to deal with this.
The first thing I try to do is to give the employee some time off. I may say something like, "Things are pretty well under control for the rest of the week and I can see that you are having some problems. Why don't you take a few days off and think about what's causing your problems.
If your shop offers paid vacations or any paid days off, you could offer a few paid days taken early, If not, the jolt of having down time with no pay might be just the thing to make the employee think about what it might be like to have no job.
Often, just getting a change of scenery or routine can be enough to get a wayward employee back on track.