How to re-enable read ahead (prefetch) feature in SQL Server after disabling it?
All you need do is turn the trace flag off globally:

DBCC TRACEOFF (652, -1);


A service restart is not required.

To control read-ahead for the current session only, simply omit the -1.

DBCC TRACEON (652);
...
...tests
....
DBCC TRACEOFF (652);


This trace flag is only minimally documented, so for anything more than educational purposes on an isolated test system, it makes sense to contact Microsoft Support for advice.

Note: QUERYTRACEON is not effective for trace flag 652.